add Perl_op_class(o) API function

add Perl_op_class(o) API function

Given an op, this function determines what type of struct it has been
allocated as. Returns one of the OPclass enums, such as OPclass_LISTOP.

Originally this was a static function in B.xs, but it has wider
applicability; indeed several XS modules on CPAN have cut and pasted it.

It adds the OPclass enum to op.h. In B.xs there was a similar enum, but
with names like OPc_LISTOP. I've renamed them to OPclass_LISTOP etc. so as
not to clash with the cut+paste code already on CPAN.