Add &CORE::undef

Add &CORE::undef

In the error message, we can’t say ‘&CORE::undef operator’, so we
should be using the op name, rather than the op description.

Instead of using OP_NAME(PL_op->op_next), which would expand to

    PL_op->op_next->op_type == OP_CUSTOM
	? XopENTRY(Perl_custom_op_xop(aTHX_ PL_op->op_next), xop_name)
	: PL_op_name[PL_op->op_next->op_type]

we can simply use PL_op_name[opnum], which should be quicker.

pp_undef can already handle nulls on the stack.

There is one remaining problem.  If &CORE::undef(\*_) is called, *_
will be undefined while @_ is localised during the sub call, so it
won’t have the same effect as undef *_.  I don’t know whether this
should be considered a bug or not, but I could solve it by making
pp_undef an XSUB.