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.

Review by @VladUreche

This will introduce a binary incompatibility in the library wrt
2.11.0-M8. We'll only notice this if partest or scalacheck calls
`andThen` or `compose` on `Function1`; our PR validation build
should tell us whether that's a problem or not.