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.

and renamed it "Core words". Also, reworded the presentation.

Finally got rid of the XPACKed core and managed to cross-compile
all core words, which greatly simplifies the bootstrapping process.

TODO: implement X['] so that I can remove those XCURRENT patterns.

This leaves space for xcomp-core which is growing.

This allows us to move words like ABORT" to xcomp-core, which is
I think the last roadblock before being able to unify all drivers
into a single xcomp layer.

And refreshed description.

With all this recent movements, we had a bit of a fragmentation
issue.