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

src/include/janet.h

@@ -645,6 +645,7 @@ struct JanetFiber {
     int32_t stacktop; /* Top of stack. Where values are pushed and popped from. */
     int32_t capacity;
     int32_t maxstack; /* Arbitrary defined limit for stack overflow */
+    JanetTable *env; /* Dynamic bindings table (usually current environment). */
     Janet *data;
     JanetFiber *child; /* Keep linked list of fibers for restarting pending fibers */
 };