Use new `Type`-based generic resolution in `write_rust_path`

This gives us better generic resolution, but because we determine
whether to include a `crate::` prefix or not based on generic
resolution it means we have to pipe through the "type is a generic that
was resolved" flag from `write_rust_type`. We also take this opporunity
to do better decision making for the `crate::` prefix entirely, which
is important for enums that contain trait instances.