Integrate perlio:

[ 14250]
Slab allocator for ops
- moved the statics to intrpvar.h
- implemented Slab_Free()
- uses PerlMemShared (for now) if distinction exists.

p4raw-link: @14250 on //depot/perlio: 238a4c30b3724d314933955c5c4a0162eae05ee0

p4raw-id: //depot/perl@14251

diff --cc perl.h
Simple merge