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

Now more low/high layers.

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...

And refreshed description.

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