Update `Erasable` impls for lists.

- Merge the `List` and `ListWithCachedTypeInfo` impls by referring to
  the shared underlying type, `RawList`. This required increasing the
  visibility of `RawList`.

- Explain why a separate impl is needed for `RawList`.

- Remove an incorrect FIXME comment. Fat reference types will always
  need a separate impl. (To be sure, one could use `T: ?Sized` to merge
  the impls for `&T` and `&[T]` and it would compile but it would behave
  incorrectly because `&T` needs one word of storage and `&[T]` needs
  two words of storage. I tried it and it did not go well.)