Investigate MOS 3.50 changes

[tom-seddon]

MOS 3.50 manual here: https://github.com/bitshifters/bbc-documents/blob/master/Master128/MOS350.txt

It's got a quite comprehensive-looking list of all the changes. Should go through and try to link the MOS 3.50 code to this list. Some of this stuff will be in MOS 5.x too.