As discussed in #11
Known limitations:
- Only supports l8ui/l16ui
- Unoptimised load routine
- Not called from DoubleExceptionHandler but should be
- Doesn't restore state properly when falling through to fatal exception handler
Mostly determined from reverse engineering and poking around.
Includes first "experiments" program with random bits and pieces for
poking at registers, may be useful to keep in source control but not
useful for writing actual programs.