In struct regexp replace the two arrays of I32s accessed via startp

In struct regexp replace the two arrays of I32s accessed via startp
and endp with a single array of struct regexp_paren_pair, which has 2
I32 members. PL_regstartp and PL_regendp are replaced with a pointer
to regexp_paren_pair. The regexp swap structure now only has one
member, so abolish it and store the pointer to the swap array directly.
Hopefully keeping the corresponding start and end adjacent in memory
will help with cache coherency.

p4raw-id: //depot/perl@30769