Could ArrayVec and ArrayString implement Ord?

[frankmcsherry]

I would love to use ArrayString, but it doesn't implement Ord which is a constraint for me (types in differential dataflow need to be Ord so they can be sorted for deduplication/consolidation).

It seems like it could be a pretty simple implementation, as String just derives Ord which means it gets the Vec<u8> implementation. I guess it would be similar here, except needing to manually slice the backing contents in the impl to avoid touching invalid data. It seems like ArrayVec could be similarly workable, for types T: Ord.

Edit: I'd be happy to do a PR, but I'm not sure I grok all of the index cleverness, and wouldn't want to mess things up. If you'd rather I take a stab first, let me know. :)