rustdoc: Inlined impls lack associated type definitions

[tomjakubowski]

For example:

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

pub struct Struct;

impl Foo for Struct {
    type Out = i32;
}

// bar.rs
pub use bar::Struct

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