substring split now uses /F's fast string matching algorithm.

substring split now uses /F's fast string matching algorithm.
  (If compiled without FAST search support, changed the pre-memcmp test
   to check the last character as well as the first.  This gave a 25%
   speedup for my test case.)

Rewrote the split algorithms so they stop when maxsplit gets to 0.
Previously they did a string match first then checked if the maxsplit
was reached.  The new way prevents a needless string search.