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optimise save/restore of PL_delaymagic.
A few places (pp_push, pp_unshift, pp_aassign) have to
set PL_delaymagic on entry, and restore it on exit. These are hot
pieces of code. Rather than using ENTER/SAVEI16(PL_delaymagic)/LEAVE,
add an extra field to the jumpenv struct, and make the JUMPENV_PUSH / POP
macros automatically save and restore this var.
This means that pp_push etc only need to do a local save:
U16 old_delaymagic = PL_delaymagic;
PL_delaymagic = DM_DELAY;
....
PL_delaymagic = old_delaymagic;
and in case of an exception being raised, PL_delaymagic still gets
restored.
This transfers the cost of saving PL_delaymagic from each call to
pp_aassign etc to each time a new run level is invoked. The latter should
be much less frequent.
Note that prior to this commit, pp_aassign wasn't actually saving and
restoring PL_delaymagic; it was just setting it to 0 at the end. So this
commit also makes pp_aassign safe against PL_delaymagic re-entrancy like
pp_push and pp_unshift already were.