Change ast::ty_param_bound so that all ty param bounds are represented
as traits, with no special cases for Copy/Send/Owned/Const.
typeck::collect generates the special cases.
A consequence of this is that code using the #[no_core] attribute
can't use the Copy kind/trait. Probably not a big deal?
As a side effect, any user-defined traits that happen to be called
Copy, etc. in the same module override the built-in Copy trait.
r=nmatsakis
Closes#2284
