remove Any[Mut]RefExt traits in favor of impl Any

[japaric]

Needs a snapshot that contains PR #20385

r? @aturon

[aturon]

This looks great to me. Did you try the same treatment in the extension trait in boxed? (Implementing it directly on Box<Any>)

[alexcrichton]

For posterity, I tried to remove BoxAny in #20420, which required some compiler changes to allow inherent impls on Box inside of liballoc.

While accepted by the compiler, it ended up rejecting calling .downcast() on Box<Any + Send> in the tests of std::thread, so I had to back that part of the change out.

[aturon]

Needs a rebase (I believe I r+'ed a previous version?)