Use EXX instead of the stack for HL protection and remove all
spurious uses of chkPS,

I wanted to inline chkPS in next because of its "tight loop" status,
but for reasons I don't understand, doing so breaks Collapse OS.
Later...

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...

Will be useful for assembling binaries for the TRS-80 which can't start
at addr 0.