This commit aligns `wiggle` a little bit closer with Rust proper in
the sense that now `GuestTypeCopy` implies `GuestTypeClone` which
in turn implies that any type implementing `GuestTypeCopy` will have
to provide `read_from_guest` and `write_to_guest` methods. As a result,
we can safely revert changes introduced in the previous commit.
a02bce6eaf