chkPS, used to be only for words that pushed back to PS, but I've
recently removed underflow checks from next and I forgot to add
missing chkPS, calls that go with this change.
DO and LOOP now only compile words from stable ABI, which make them
suitable for xcomp. This will greatly simplify driver writing and
allow us to mostly avoid the low/high divide in drivers.
LEAVE is still xcomp-incompatible though.
This make us very very tight for "<0x100" limit for literal words.
Hopefully, we won't need more stuff in that area of the binary...
