-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.