Prevent half-specialization

[andrew-johnson-4]

Problem:

• open : x -> x
• open : U64 -> U64
• applying open U8 will unify with open U64 first rather than the more specific specialization that would result from the parameterized version open x

This ticket is blocked by garbage collection because the naive algorithm here is a bit expensive.

Is there a good algorithm here that isn't too costly?