Clone for into-iterators

[clarfonthey]

The hashbrown into-iterators currently don't support Clone (see: rust-lang/hashbrown#545) but this shouldn't stop this crate from implementing it for its into-iterators, which ditch the hash table and just use vec::IntoIter internally.

[cuviper]

Sure, we could do that!

[cuviper]

One thing that's kind of weird is that IntoKeys will still need V: Clone, and IntoValues will need K: Clone, so we can have a complete clone of the underlying vec::IntoIter. We could possibly use Default for the wasted part instead, but that seems even weirder as a constraint. What do you think?

[clarfonthey]

Hmm, that's actually an interesting point. It'd require unsafe code, but perhaps we could transmute to make the unused parts MaybeUninit, since we're just going to ignore their values anyway, and then set them to uninit?

[cuviper]

#348 is adding the easy ones for IntoIter, but we can leave this open for IntoKeys and IntoValues.