[WIP] Implements P1207

[cor3ntin]

This implements move-only iterators. I still have to write tests for 35 or so more algorithms accepting InputIterator.

[CaseyCarter]

I rebased this onto master, refactored ext::uncounted/ext::recounted to require rvalues, and tweaked some things. Let me know / point me at a branch if I've created conflicts for you, and I'll rebase your work onto this. (I have a bad habit of breaking things, but I don't mind cleaning up my own mess.)

I still have to write tests for 35 or so more algorithms accepting InputIterator.

Don't forget the algorithms that accept OutputIterator / WeaklyIncrementable. We've allowed all single-pass iterators to be move-only, not just InputIterators.

[cor3ntin]

@CaseyCarter Thanks a lot - i feel bad... I'll try to make some progress this week. Yes, i was counting all relevant algorithms, including a few OutputIterators (in fill, fill_n and generate, I may miss a few)