Integrate object projection predicates into rustdoc

[nikomatsakis]

I think an object type like Iterator<T=uint> currently just ignores the T=uint bounds found in the ExistentialBounds struct.

[nikomatsakis]

This is particularly relevant if we move function return types to be an associated type as proposed in #20871.

[nikomatsakis]

These are ignored for where-clauses too, afaik.

[tomjakubowski]

This is sort of done, though awkwardly for re-export cases, for where clauses after #21018. It should already be working for where clauses in signatures on items defined in the crate rustdoc is working on, like for:

pub trait Trait {
    type Out;
}

pub fn foo<T>(_: T) where T: Trait<Out=i32> { }

[tomjakubowski]

As for object projection predicates, this should work already for the "one crate" case, but not for re-exports (as usual!).