Avoid needless TypeRef allocation during erasure.

- some.package.type need not be erased. We avoid this
    by sharpening the check in `rebindInnerClass`. The answer
    to the deleted comment in the code, BTW, is anonymous and
    local classes, specifically excluded in the new formulation.
  - detect if erasure of the TypeRef is an identity and reuse
    the original.

Waste not, want not.