Splitting a word into MSB/LSB pairs happens often and is worth, I
think, native words. Also, I'm going to need it in the upcoming
commits.

Recipes contain bits and pieces of hardware-related knowledge, but
these bits feel sparse. I've been wanting to consolidate hardware-
related documentation for a while, but always fell at odds with the
recipes organisation.

We don't have recipes anymore, just a /doc/hw section that contains
hardware-related documentation which often translate to precise
instructions to run Collapse OS on a specific machine.

With this new organisation, I hope to end up with a better, more
solid documentation.

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.

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

Makes xcomp.fs units shorter.

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

Now more low/high layers.

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.

Now I struggle to remember why I ever did things they way I did.
There must have been a reason...

The goal is to insert device drivers in between.

I'd like to move this code, along with all recipe contents, to blk/.
It's easier if "emul-related" code is gathered at the same place.

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

Not worth the complexity and space.

XPACK is more efficient than stripfc was, we can pack more stuff in 8K.
We now have enough space to fit readln.

Big bootstrapping change which greatly reduces the role of the modern
operating system in the bootstrapping process.

The way is clear for complete stage1 bootstrapping on the RC2014
target!