add a fast O(n log n) stable sort without Clone to std::vec

[thestinger]

The extra::sort module can be removed when this is done. A simple merge sort making use of insertion sort for small runs is good enough for everyone! Since it will be simple, making use of some unsafe code for speed will be fine.

If allocation isn't acceptable, PriorityQueue can be leveraged to do an in-place heap sort and will likely end up in std too.