Figure out why tags get rebuilt in compares-not-equal order, and fix

[graydon]

It seems that we sometimes rebuild tags during typechecking in a way that causes the constructors to reorder, and tag-equality comparison fails (in the ocaml sense). Figure out why or how this happens and fix it. Currently hacked-around.

[graydon]

Canonicalize hashtables after running them through htab_map. Closed by 329a655.