Fix a couple compile warnings
CT substitute unqualified true/false/null in namespaces

Substituation now always happens on the resolved name and
unqualified usages of true/false/null in namespaced code are
substituted as well.
Drop unused RAISE_ABSTRACT_ERROR opcode

Abstract methods are being prevented from being called in DO_FCALL
etc.
Drop unused INIT_STRING opcode

INIT_STRING has been implemented as an UNUSED op1 to ADD_* for
some time now.