Specific implementations on wrapper types should show up on the page of the inner type

[Manishearth]

Currently, with the following code:

pub struct Rc<T>(T);

pub struct Foo;


impl Rc<Foo> {
    pub fn bar() {}
}

the impl appears on the page for Rc, even though it's more relevant for Foo.

Provide a way to mark a type as a wrapper type, such that an impl where its first generic parameter is a single type (which may itself be generic, e.g. Rc<Option<T>>) appears on the page of that type.

It may be worth automatically doing this for #[fundamental] types