Parameterize shim trait impl blocks over lifetimes

[dtolnay]

Part of #608. This updates all Rust generated code when instantiating generic types containing lifetime parameters. This builds on the lifetimes collected by the name resolver in #634.

#[cxx::bridge]
mod ffi {
    extern "C++" {
        type Borrowed<'a>;
                     ^^^^
        fn demo<'b>(s: &'b str) -> UniquePtr<Borrowed<'b>>;
    }
}

turns into:

unsafe impl<'a> UniquePtrTarget for Borrowed<'a> {...}
           ^^^^                             ^^^^