Stabilizing basic functions on arrays and slices

[bluss]

@huon

Yes, and as I said there's no way to change the implementation to get the guarantees without specialisation.

But we might have specialization by provisional and ad-hoc means as a crutch. We already have std::intrinsics::needs_drop, we could also have an intrinsic to tell if a type is Copy, so this is very far from impossible.