Explicitly requesting an instantiation

[dtolnay]

As discussed in #312 and elsewhere. Supersedes #329. We use the following impl syntax to indicate that we'd like the expansion of the ffi mod to include a given instantiation of UniquePtr, even if otherwise not mentioned in any of the extern function signatures:

#[cxx::bridge]
mod ffi {
    extern "C" {
        type C;
    }

    impl UniquePtr<C> {}
}

The same also works if the UniquePtr were to be an extern type alias (type C = path::to::C;). In that case it's up to the user whether to place the UniquePtr impl in one ffi module or the other, subject to the Rust orphan rule.

Closes #236. Closes #312. Closes #329.