Move coresub op-creation from gv.c to op.c

Move coresub op-creation from gv.c to op.c

For functions that take handles as arguments, this code will need to
call static functions in op.c, like is_handle_constructor.

While we could make is_handle_constructor into a non-static function
and call it from gv.c, that seems backwards, as it would result in a
lot of op-manipulation code in the middle of gv.c.

So this commit creates a new function in op.c, called coresub_op,
which is only called from gv.c, from the &CORE::sub code.