Review MESS code to see if em-dosbox's learnings can apply

[vitorio]

https://github.com/dreamlayers/em-dosbox/ is an emscripten port of DOSbox, which reveals two interesting optimizations:

https://gist.github.com/dreamlayers/8463670 as discussed here: emscripten-core/emscripten#1909

and https://github.com/dreamlayers/em-dosbox/blob/em-dosbox-0.74/src/cpu/extractfun.py as discussed in the fourth bullet point here: https://github.com/dreamlayers/em-dosbox/blob/em-dosbox-0.74/README.md#other-issues

@DopefishJustin tried applying the former patch but it didn't work without tweaks, and he didn't notice an appreciable speedup.

It would be interesting to test it more thoroughly, and to review MESS code to see if the latter method could be applied, and if it results in a speedup.