Add (VERY) simple closed-form reduce simplification code.

This pretty much only recognises the kind of reduces and redomaps
created by size analysis.  In time, perhaps we can make it more
powerful, but it is not a simple problem at all.
Re-add some simplification rules that had become lost in the great refactor.

The important thing is that expressions like 'e1==e2' will evaluate to
True if e1 and e2 have the same representation (e.g. they are the same
variable).