Update tracev macro to be simpler and single arity.

Reference #401

CHANGELOG.md

@@ -2,6 +2,7 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased - ???
+- Add `tracev` macro.
 - Fix compiler bug that emitted incorrect code in some cases for while loops that create closures.
 - Add `:fresh` option to `(import ...)` to overwrite the module cache.
 - `(range x y 0)` will return an empty array instead of hanging forever.

src/boot/boot.janet

@@ -947,30 +947,14 @@
   (tuple 'fn (tuple '& $args) (tuple/slice parts 0)))
 
 (defmacro tracev
-  "Displays the variables or literals listed, providing both the name and
-  and the value for variables. Designed for quick debugging of values. Returns
-  the traced forms: nil for none, the form itself for one, and a tuple of the
-  forms for many."
-  [& forms]
-  (with-syms [results var]
-    ~(do
-      (def ,results @[])
-      ,;(map (fn [form]
-               ~(do
-                 (def ,var ,form)
-                 (eprintf (string "trace "
-                                  (dyn :pretty-format "%q")
-                                  " is "
-                                  (dyn :pretty-format "%q"))
-                          ',form
-                          ,var)
-                 (eflush)
-                 (array/push ,results ,var)))
-             forms)
-      (case (length ,results)
-        0 nil
-        1 (,results 0)
-        (tuple ;,results)))))
+  "Print a value and a description of the form that produced that value to
+  stderr. Evaluates to x."
+  [x]
+  (def s (gensym))
+  ~(let [,s ,x]
+     (,eprinf "trace %j is " ',x)
+     (,eprintf (,dyn :pretty-format "%q") ,s)
+     ,s))
 
 (defmacro ->
   "Threading macro. Inserts x as the second value in the first form