&CORE::lock()

&CORE::lock()

This commit allows &CORE::lock to be called through references and
via ampersand syntax.  It adds code to pp_coreargs for handling the
OA_SCALARREF case, though what it adds is currently lock-specific.
(Subsequent commits will address that.)  Since lock returns the scalar
passed to it, not a copy, &CORE::lock needs to use op_leavesublv,
rather than op_leavesub.  But it can’t be an lvalue sub, as
&CORE::lock = 3 should be disallowed.  So we use the sneaky trick of
turning on the lvalue flag before attaching the op tree to the sub
(which causes newATTRSUB to use op_leavesublv), and then turning it
off afterwards.