Add support for quantified bounds on GATs

[scalexm]

Closes #116 modulo #144.

[nikomatsakis]

Can we get a test where the bound would've worked, but for the higher-ranked nature of it? I am thinking of something like:

struct foo;
impl Fn<i32, i32> for foo { }

impl Bar for i32 {
  type Item<T> = foo;
}

[scalexm]

@nikomatsakis I forgot to add an impl<T, U> Fn<T, U> for fn<T, U> { } in that test, I think the (now) corrected test is what you want