Fix #12442.

The problem is described in the ticket.

This patch adds new variants of the access points to the pure
unifier that allow unification of types only when the caller
wants this behavior. (The unifier used to also unify kinds.)
This behavior is appropriate when the kinds are either already
known to be the same, or the list of types provided are a
list of well-typed arguments to some type constructor. In the
latter case, unifying earlier types in the list will unify the
kinds of any later (dependent) types.

At use sites, I went through and chose the unification function
according to the criteria above.

This patch includes some modest performance improvements as we
are now doing less work.