With the release of arrayvec 0.5, these crates are unused.
With the release of Rust 1.36 and MaybeUninit, these crates have no
purpose (but for older Rust releases).
Clone is not so important, but Copy is, since it's structural when
implemented, and adding the trait allows more versatile use of
NoDrop<T>.
Only the untagged unions version can implement Copy for NoDrop<T>, since
it truly has no destructor.