This is a complete rewrite of the standard sort algorithm. The new algorithm
is a simplified variant of TimSort. In summary, the changes are:

* Improved performance, especially on partially sorted inputs.
* Performs less comparisons on both random and partially sorted inputs.
* Decreased the size of temporary memory: the new sort allocates 4x less.

Since merge_sort is generic and collapse isn't, that means calls to
collapse won't be inlined.  inlined. Therefore, we must stick an
`#[inline]` above `fn collapse`.