Teach as_mast_constant about QAST::Want.
Teach as_mast_constant about QAST::Want.

Makes some of the nqp::ops usable from Rakudo that previously weren't.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp