Add defdyn macro to allow docs and checking for dyns.

Using keywords for the names of dynamic bindings emphasized their
dynamic nature and how they actually work, but is opaque when it comes
to documentation and error detection. Janet uses early binding for name
resolution by default in most places, dyns should be no different.

The `defdyn` macro allows one to create aliases for keywords that can
have docstrings, be imported and exported, etc. The aliases _must_
follow the usual lisp convention of earmuffs - this is not
restricting since the underlying keyword lookup mechanism is still
completely accessible to users.

Example:

(defdyn *my-dynamic-binding* "Sends the plumbus to the thingamizer when
 enabled")

The above creates a normal binding (as created with `def`) for
`*my-dynamic-binding*` that is bound to the keyword
`:my-dynamic-binding`.

There is an optional prefix for defdyns that can be used to avoid name
collisions - *defdyn-prefix*

Example:

(setdyn *defdyn-prefix* "mylib/")
(defdyn *my-dynamic-binding* "Plumbus thingamizer")
(pp *my-dynamic-binding*)

> :mylib/my-dynamic-binding

src/boot/boot.janet

@@ -1155,12 +1155,36 @@
     (array/push parts (tuple apply f $args)))
   (tuple 'fn (tuple '& $args) (tuple/slice parts 0)))
 
+(defmacro defdyn
+  ``Define an alias for a keyword that is used as a dynamic binding. The
+  alias is a normal, lexically scoped binding that can be used instead of
+  a keyword to prevent typos. Defdyn does not set dynamic bindings or otherwise
+  replace `dyn` and `setdyn`. The alias _must_ start and end with the `*` character, usually
+  called "earmuffs".``
+  [alias & more]
+  (assert (symbol? alias) "alias must be a symbol")
+  (assert (and (> (length alias) 2) (= 42 (first alias) (last alias))) "name must have leading and trailing '*' characters")
+  (def prefix (dyn :defdyn-prefix))
+  (def kw (keyword prefix (slice alias 1 -2)))
+  ~(def ,alias :dyn ,;more ,kw))
+
+(defdyn *defdyn-prefix* "Optional namespace prefix to add to keywords declared with `defdyn`.
+  Use this to prevent keyword collisions between dynamic bindings.")
+(defdyn *out* "Where normal print functions print output to.")
+(defdyn *err* "Where error printing prints output to.")
+
+(defdyn *macro-form*
+  "Inside a macro, is bound to the source form that invoked the macro")
+
+(defdyn *current-file*
+  "Bound to the name of the currently compiling file.")
+
 (defmacro tracev
   `Print a value and a description of the form that produced that value to
   stderr. Evaluates to x.`
   [x]
-  (def [l c] (tuple/sourcemap (dyn :macro-form ())))
-  (def cf (dyn :current-file))
+  (def [l c] (tuple/sourcemap (dyn *macro-form* ())))
+  (def cf (dyn *current-file*))
   (def fmt-1 (if cf (string/format "trace [%s]" cf) "trace"))
   (def fmt-2 (if (or (neg? l) (neg? c)) ":" (string/format " on line %d, column %d:" l c)))
   (def fmt (string fmt-1 fmt-2 " %j is "))
@@ -1655,10 +1679,13 @@
   (file/close f)
   nil)
 
+(defdyn *pretty-format*
+  "Format specifier for the `pp` function")
+
 (defn pp
-  `Pretty print to stdout or (dyn :out). The format string used is (dyn :pretty-format "%q").`
+  `Pretty print to stdout or (dyn *out*). The format string used is (dyn *pretty-format* "%q").`
   [x]
-  (printf (dyn :pretty-format "%q") x)
+  (printf (dyn *pretty-format* "%q") x)
   (flush))
 
 ###
@@ -1882,13 +1909,17 @@
 ###
 ###
 
+(defdyn *macro-lints*
+  "Bound to an array of lint messgae that will be reported by the compiler inside a macro.
+  To indicate an error or warning, a macro author should use `maclintf`.")
+
 (defn maclintf
   ``When inside a macro, call this function to add a linter warning. Takes
   a `fmt` argument like `string/format` which is used to format the message.``
   [level fmt & args]
-  (def lints (dyn :macro-lints))
+  (def lints (dyn *macro-lints*))
   (when lints
-    (def form (dyn :macro-form))
+    (def form (dyn *macro-form*))
     (def [l c] (if (tuple? form) (tuple/sourcemap form) [nil nil]))
     (def l (if-not (= -1 l) l))
     (def c (if-not (= -1 c) c))
@@ -1983,7 +2014,7 @@
     (def m? (in entry :macro))
     (cond
       s (s t)
-      m? (do (setdyn :macro-form t) (m ;(tuple/slice t 1)))
+      m? (do (setdyn *macro-form* t) (m ;(tuple/slice t 1)))
       (tuple/slice (map recur t))))
 
   (def ret
@@ -2191,10 +2222,13 @@
   (def newenv (table/setproto @{} parent))
   newenv)
 
+(defdyn *err-color*
+  "Whether or not to turn on error coloring in stacktraces and other error messages.")
+
 (defn bad-parse
   "Default handler for a parse error."
   [p where]
-  (def ec (dyn :err-color))
+  (def ec (dyn *err-color*))
   (def [line col] (:where p))
   (eprint
     (if ec "\e[31m" "")
@@ -2231,7 +2265,7 @@
 (defn warn-compile
   "Default handler for a compile warning"
   [msg level where &opt line col]
-  (def ec (dyn :err-color))
+  (def ec (dyn *err-color*))
   (eprin
     (if ec "\e[33m" "")
     where
@@ -2249,7 +2283,7 @@
 (defn bad-compile
   "Default handler for a compile error."
   [msg macrof where &opt line col]
-  (def ec (dyn :err-color))
+  (def ec (dyn *err-color*))
   (eprin
     (if ec "\e[31m" "")
     where
@@ -2329,7 +2363,7 @@
   (var where default-where)
 
   (if (string? where)
-    (put env :current-file where))
+    (put env *current-file* where))
 
   # Evaluate 1 source form in a protected manner
   (def lints @[])
@@ -2411,7 +2445,7 @@
       (do
         (set where new-where)
         (if (string? new-where)
-          (put env :current-file new-where)))
+          (put env *current-file* new-where)))
 
       (do
         (var pindex 0)
@@ -2669,7 +2703,7 @@
       (os/exit 1))
     (put env :exit true)
     (def buf @"")
-    (with-dyns [:err buf :err-color false]
+    (with-dyns [*err* buf *err-color* false]
       (bad-parse x y))
     (set exit-error (string/slice buf 0 -2)))
   (defn bc [&opt x y z a b]
@@ -2678,7 +2712,7 @@
       (os/exit 1))
     (put env :exit true)
     (def buf @"")
-    (with-dyns [:err buf :err-color false]
+    (with-dyns [*err* buf *err-color* false]
       (bad-compile x nil z a b))
     (set exit-error (string/slice buf 0 -2))
     (set exit-fiber y))
@@ -2827,6 +2861,12 @@
   [&opt env local]
   (env-walk keyword? env local))
 
+(defdyn *doc-width*
+  "Width in columns to print documentation printed with `doc-format`")
+
+(defdyn *doc-color*
+  "Whether or not to colorize documentation printed with `doc-format`.")
+
 (defn doc-format
   `Reformat a docstring to wrap a certain width. Docstrings can either be plaintext
   or a subset of markdown. This allows a long single line of prose or formatted text to be
@@ -2834,10 +2874,10 @@
   [str &opt width indent colorize]
 
   (default indent 4)
-  (def max-width (- (or width (dyn :doc-width 80)) 8))
+  (def max-width (- (or width (dyn *doc-width* 80)) 8))
   (def has-color (if (not= nil colorize)
                    colorize
-                   (dyn :doc-color)))
+                   (dyn *doc-color*)))
 
   # Terminal codes for emission/tokenization
   (def delimiters
@@ -3200,7 +3240,7 @@
   "Print the current fiber stack"
   []
   (print)
-  (with-dyns [:err-color false] (debug/stacktrace (.fiber) (.signal) ""))
+  (with-dyns [*err-color* false] (debug/stacktrace (.fiber) (.signal) ""))
   (print))
 
 (defn .frame
@@ -3554,6 +3594,7 @@
      (debug/stacktrace f e "")))
   nil)
 
+
 ###
 ###
 ### CLI Tool Main
@@ -3570,12 +3611,22 @@
     (let [thunk (compile [main ;subargs] env arg)]
       (if (function? thunk) (thunk) (error (thunk :error))))))
 
+(defdyn *args*
+  "Dynamic bindings that will contain command line arguments at program start")
+
+(defdyn *executable*
+  "Name of the interpreter executable used to execute this program. Corresponds to argv[0] in the call to
+   int main(int argc, char **argv);")
+
+(defdyn *profilepath*
+  "Path to profile file loaded when starting up the repl.")
+
 (defn cli-main
   `Entrance for the Janet CLI tool. Call this function with the command line
   arguments as an array or tuple of strings to invoke the CLI interface.`
   [args]
 
-  (setdyn :args args)
+  (setdyn *args* args)
 
   (var should-repl false)
   (var no-file true)
@@ -3591,7 +3642,7 @@
   (var expect-image false)
 
   (if-let [jp (getenv-alias "JANET_PATH")] (setdyn :syspath jp))
-  (if-let [jprofile (getenv-alias "JANET_PROFILE")] (setdyn :profilepath jprofile))
+  (if-let [jprofile (getenv-alias "JANET_PROFILE")] (setdyn *profilepath* jprofile))
 
   (defn- get-lint-level
     [i]
@@ -3601,7 +3652,7 @@
   # Flag handlers
   (def handlers
     {"h" (fn [&]
-           (print "usage: " (dyn :executable "janet") " [options] script args...")
+           (print "usage: " (dyn *executable* "janet") " [options] script args...")
            (print
              ```
              Options are:
@@ -3663,7 +3714,7 @@
      "d" (fn [&] (set debug-flag true) 1)
      "w" (fn [i &] (set warn-level (get-lint-level i)) 2)
      "x" (fn [i &] (set error-level (get-lint-level i)) 2)
-     "R" (fn [&] (setdyn :profilepath nil) 1)})
+     "R" (fn [&] (setdyn *profilepath* nil) 1)})
 
   (defn- dohandler [n i &]
     (def h (in handlers n))
@@ -3719,7 +3770,7 @@
           (file/flush stdout)
           (file/read stdin :line buf))
         (def env (make-env))
-        (when-let [profile.janet (dyn :profilepath)]
+        (when-let [profile.janet (dyn *profilepath*)]
             (def new-env (dofile profile.janet :exit true))
             (merge-module env new-env "" false))
         (when debug-flag