rustdoc: Inlined impls lack associated type definitions

For example:

``` rust
// foo.rs
pub trait Foo {
    type Out;
}

pub struct Struct;

impl Foo for Struct {
    type Out = i32;
}
```

``` rust
// bar.rs
pub use bar::Struct
```

The "trait implementations" section for `bar::Struct` will show `impl Foo for Struct` with no associated type.
