Refactor Sequence::sort

[gdejohn]

Sequence::sort implements the basic "functional quicksort" (more precisely, deforested tree sort), which is O(n log n) on average but O(n^2) worst case. This should be refactored with an algorithm that is adaptive and O(n log n) worst case, but still purely functional, stable, and lazy so that it can start producing elements before the entire sequence has been sorted. Perhaps natural merge sort?