Many changes for adding dynamic (fiber-level) scope.

- Allow passing a table to fibers, which make fiber level scope easier.
- Add fiber/getenv, fiber/setenv, dyn, and setdyn
- Remove meta, *env*, and *doc-width*
- Some functions changed dignatures, and no longer take an env

tools/gendoc.janet

@@ -103,7 +103,7 @@
 
 # Generate parts and print them to stdout
 (def parts (seq [[k entry]
-                 :in (sort (pairs (table/getproto *env*)))
+                 :in (sort (pairs (table/getproto (fiber/getenv (fiber/current)))))
                  :when (and (get entry :doc) (not (get entry :private)))]
                 (emit-item k entry)))
 (print