Testing:
o  Giant runs.
o  Records of odd lengths.
o  Empty files.

Improvements:
o  Alignment? Use of SSE?
o  Use radix-sort for internal sorting.
o  Parallelization of internal sorting.
o  Clean up data types and make sure they cannot overflow. (size_t vs. u64 vs. sh_off_t vs. uns)
o  Buffer sizing in internal sorters.
o  Switching between direct and normal I/O.
o  When merging, choose the output file with less runs instead of always switching?
o  Implement multi-way merge.
o  Mode with only 2-way unification?
o  Speed up 2-way merge.
o  Speed up radix splitting.
o  A debug switch for disabling the presorter.