In a recent survey, three out of four cross assemblers surveyed
recommended not using opcode mnemonics to their patients who use
labels. We now remap labels like "AND" and "jmp", using the label
map that's part of the label localizer.
We skip the step for Merlin 32, which is perfectly happy to assemble
"JMP JMP JMP".
Also, fixed a bug in MaskLeadingUnderscores that could hang the
source generator thread.