Note [tcMatchTy vs tcMatchTyKi]

This module offers two variants of matching: with kinds and without.
The TyKi variant takes two types, of potentially different kinds,
and matches them. Along the way, it necessarily also matches their
kinds. The Ty variant instead assumes that the kinds are already
eqType and so skips matching up the kinds.

How do you choose between them?

1. If you know that the kinds of the two types are eqType, use
   the Ty variant. It is more efficient, as it does less work.

2. If the kinds of variables in the template type might mention type families,
   use the Ty variant (and do other work to make sure the kinds
   work out). These pure unification functions do a straightforward
   syntactic unification and do no complex reasoning about type
   families. Note that the types of the variables in instances can indeed
   mention type families, so instance lookup must use the Ty variant.

   (Nothing goes terribly wrong -- no panics -- if there might be type
   families in kinds in the TyKi variant. You just might get match
   failure even though a reducing a type family would lead to success.)

3. Otherwise, if you're sure that the variable kinds do not mention
   type families and you're not already sure that the kind of the template
   equals the kind of the target, then use the TyKi version.