Overhaul of the rewrite rules

Overhaul of the rewrite rules
- Cleaned up and simplified rules
- Added many missing cases
- The rules OccursCheck and swap have been eliminated and integrate with
  the other rules; ie, Subst and Unify perform the occurs check themselves
  and they can deal with left-to-right and right-to-left oriented rewrites.
  This makes the code simpler and more efficient.
- Also added comments.