System variables

There are some core variables in the core system that are
referred to directly by their address in memory throughout the
code. The place where they live is configurable by the RAMSTART
constant in conf.fs, but their relative offset is not. In fact,
they're mostly referred to directly as their numerical offset
along with a comment indicating what this offset refers to.

This system is a bit fragile because every time we change those
offsets, we have to be careful to adjust all system variables
offsets, but thankfully, there aren't many system variables.
Here's a list of them:


                                                        (cont.)
