This will allow us to implement AT-XY, paving the way to all sorts
of nice new things.

It doesn't change anything featurewise, but this change is in
preparation for the addition of an eventual AT-XY.

It's not used

To update LATEST offset more easily. Will be useful for recipes.

This requires us to significantly adjust our build process, which
now has 3 stages.

fixes #80

Also, add the new `bshell` emulated tool. BASIC is on its way to replace the
shell.

fixes #79