Enumeration set universe comparisons should be done with `equal?'

Enumeration set universe comparisons should be done with `equal?'

* module/rnrs/enums.scm (enum-set-union, enum-set-intersection,
  enum-set-difference): Compare enum-set universes with `equal?' to support
  sets generated using constructor syntax bound by `define-enumeration'.
* test-suite/tests/r6rs-enums.test (enum-set-union, enum-set-intersection,
  enum-set-difference): New test cases for syntactically-generated sets.