Include empty symbol in variable enumeration

Please merge into 6.0.
Initial version of enumeration docs

(Please include on the release branch --robby)
Misc redex enumerator tweaks:

- edits to the docs
- adjust the #:i-th argument interpretation to allow
  any natural number (using modulo for finite enumerations)
- remove unnecessary gui dependency of a few test files

Please include on the release branch