Although the SPI Relay driver is RC2014-specific, the SD Card driver
is generic enough to be a subsystem. That's the second subsystem we
add and this warrants, I think, the formalization of a new concept:
protocols.
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...
