Support different namespaces on different extern blocks

[dtolnay]

Currently the namespace is determined globally for an entire #[cxx::bridge] invocation.

#[cxx::bridge(namespace = path::to)]
mod ffi {...}

It would be good to be able to pull in things from multiple different namespaces at once. Something like:

#[cxx::bridge]
mod ffi {
    #[namespace = path::to::first]
    extern "C" {
        type First;
    }

    #[namespace = path::to::second]
    extern "C" {
        type Second;
    }

    #[namespace = path::to::third]
    extern "Rust" {
        fn wow(first: &First, second: &Second);
    }
}

[dtolnay]

Implemented as part of #370.