Also made all LValues also RValues for brevity (so INC/DEC don't have
to bound their destination arguments by both, but this is not required
if it causes problems for some reason down the road).
This also promotes CALL/RET to actually using the bus for
addressing (because it goes through the generic PUSH/POP code), which
is probably more correct anyway.
Likely to be a bugfest.
There's a flags macro for now but unclear how useful it will turn out
to be in practice. Flags in particular need testing.
Also disappointing that I couldn't make instructions generic across
u8/u16, especially for ordinary, non-string CMP. Need to look into
this more to see if it can be done with num-traits or some other
trickery.
