Emit lifetimes on extern type into Rust macro output

[dtolnay]

Part of #608.

An extern type type Example<'a, 'b>; turns into:

#[repr(C)]
pub struct Example<'a, 'b> {
    _private: ::cxx::private::Opaque,
    _lifetime_a: ::std::marker::PhantomData<&'a ()>,
    _lifetime_b: ::std::marker::PhantomData<&'b ()>,
}

unsafe impl<'a, 'b> ::cxx::ExternType for Asdf<'a, 'b> {
    type Id = ::cxx::type_id!("Example");
    type Kind = ::cxx::kind::Opaque;
}

Note that we assume covariant lifetimes. It's possible we'll also expose contravariant and invariant lifetimes something like the ghost crate (https://github.com/dtolnay/ghost#variance).