Merge branch 'riastradh-20190210-letrecdefn'

- Improve performance of LETREC* and unary LETREC.
- Convert various multi-way LETRECs to internal definitions.

The compiler knows about internal definitions.  Teaching to recognize
patterns of LET and SET! produced for LETREC semantics is a lot of
trouble, and internal definitions are usually much more legible
anyway.

This is a lot of little commits in case I made a mistake so you can
bisect to find the mistake.