SI-7700 @unspecialized, Part Deux: Now Working.

This annotation was introduced to allow us to mark methods within
a specialized trait as immune from specialization. In particular,
this is desirable for `Function1.{andThen, compose}`.

However, it seems we need to check for this in two places in the
specialization code base. The feature is now backed with a test.
Merge pull request #3425 from retronym/ticket/7700

SI-7700 @unspecialized, Part Deux: Now Working.