p5-B-COW

Additional B helpers to check COW status

B::COW provides some naive additional B helpers to check the COW status of one SvPV. A COWed SvPV is sharing its string (the PV) with other SvPVs. It's a (kind of) Read Only C string, that would be Copied On Write (COW). More than one SV can share the same PV, but when one PV need to alter it, it would perform a copy of it, decrease the COWREFCNT counter. One SV can then drop the COW flag when it's the only one holding a pointer to the PV. The COWREFCNT is stored at the end of the PV, after the the "\0". That value is limited to 255, when we reach 255, a new PV would be created,