Avoid needless TypeRef allocation during erasure.

  - ThisType(some.package) need not be erased, we can let
    that prefix through unchanged and avoid churning through
    allocations.
  - Sharpen the condition in `rebindInnerClass`. The answer
    to the deleted comment in the code, is "anonymous and
    local classes", which are specifically excluded in the
    new formulation.
  - Finally, detect if erasure of the TypeRef is an identity and reuse
    the original.

Waste not, want not.

To expand on the first point, here is what used to happen during
erasure:

    scala> val scalaPack = typeOf[Predef.type].prefix
    scalaPack: $r.intp.global.Type = scala.type

    scala> typeDeconstruct.show(scalaPack)
    res19: String = ThisType(package scala)

    scala> typeDeconstruct.show(erasure.scalaErasure(scalaPack))
    res20: String = TypeRef(TypeSymbol(final class scala extends ))

Showing one step of the erasure type map:

    scala> typeDeconstruct.show(scalaPack.asInstanceOf[SubType].underlying.typeOfThis)
    res21: String = SingleType(pre = ThisType(package <root>), package scala)