Parse namespace attribute on foreign mods

[dtolnay]

This is called out in #370 as not implemented yet.

It should achieve points 1-3 on the plan in #353, and most of #​4 (it doesn't currently look for namespace attributes on extern blocks but the rest is done).

This PR implements #[namespace = "..."] attributes on extern blocks, such that an item will inherit the namespace specified on its surrounding extern block if any, otherwise the namespace specified with the top level cxx::bridge attribute.

#[cxx::bridge(namespace = "third_priority")]
mod ffi {
    #[namespace = "second_priority"]
    extern "Rust" {
        fn f();

        #[namespace = "first_priority"]
        fn g();
    }

    extern "Rust" {
        fn h();
    }
}

The above would result in functions ::second_priority::f, ::first_priority::g, ::third_priority::h.