Comment space.
Various changes:

- New macro expand functions in sandbox: `exp`, `exp+`, and `exp!`. Wrap
  them in same 3 flavors as eval -- region, definition, and last-sexp.

- Reorganize menu.

- Show full "stack trace" in error messages.

- Move some ,command code from the `syntax-case` to their own functions.

- Add `define/with-syntax`.

- Add indent and font-lock for a few forms that aren't actually in
  Racket, but rather have a Clojure flavor: `def`, `defn`, `fn`.
Fix indent for with-syntax and with-syntax*
Fix typo in comment.
Font-lock: Fix #lang line. Add module variants.

And be consistent among them:

- A language gets font-lock-keyword-face.

- A module's name gets font-lock-function-face. (Although it could be
font-lock-variable-face, IMHO a module definition is more like a
function definition than a variable definition.)