Consider confirming that the `TypeId`s used in a crate are unique

[huonw]

If two TypeIds of different types collide, we can get unsoundness, since Any relies on TypeId being unique for safe downcasts. We could have the compiler double check that any TypeIds it computes haven't occured before from another type.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#664