Implement the sort pragma. Split sort code from pp_ctl.c

Implement the sort pragma.  Split sort code from pp_ctl.c
to pp_sort.c.  Includes the quicksort stabilizing layer
from John P. Linderman.  -Msort=qsort or -Msort=fast is
faster than without (or with -Msort=mergesort or -Msort=safe)
for short random inputs, but for some reason not quite as fast
as 5.6.1 qsort.  More benchmarking, profiling, tuning, and
optimizing definitely needed.

p4raw-id: //depot/perl@13179