[Merge] New COW mechanism

This branch makes string copying faster by introducing a new copy-on-
write mechanism.  A reference count for the string buffer is now
stored inside the string buffer itself.  This can be disabled with
-Accflags=-DPERL_NO_COW.

It also disables the PL_sawampersand mechanism, as copy-on-write
can now be used for the pre-match ‘copy’, resulting in no slow-
down from using $& and f(r)iends.  This can be reënabled with
-Accflags=-DPERL_SAWAMPERSAND.