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 "\0".
 .
 That value is limited to 255, after that a new PV would be created,