addm`exceptions

1.20: addm/exceptions:

. I first had the idea of stacking
more than the usual return address,
for the 2 cases of whether or not
the system was in an exception-raised mode .
. however, in recursive situations
that would use a lot more memory;
ie, an extra pointer per call,
when the alternative is to have a
conditional at the call's return point
which could be shared by all those recursive calls .
. then I found a way in which
it wouldn't even need the conditional.stmt:
. for a call.stmt that may involve an exception,
place a goto.stmt after it;
then make the call's return address
to land beyond the goto.stmt .
. in an exception.mode, the system would
treat the return address differently,
decrementing it before using it,
which would then cause the return to
land on the goto that would jump to the
exception-casing stmt,
or if letting a super.routine handle it,
the goto could just land at the
[cleanup and exit].section .