Wrap the return value of the type_id intrinsic in an opaque box

[alexcrichton]

Right now the return value is a raw u64, but we may want to change the representation over time (ala #10389).

We should wrap the return value in a struct TypeId { priv ... } and define the Eq trait on this structure.

Fairly easy bug, but requires probably adding a lang item and mucking around with some intrinsics, so not flagging as E-easy.

[pnkfelix]

Accepted for P-high.

[Kimundi]

Note that any.rs defines just such a type, it would just need to be used by the type_id intrinsic