Cleanup of codebase and initial commit of 56K assembler by ggn.

There's still a ways to go before this will work properly as we need to
add proper fixup handling and origin (".org") bookkeeping. As it is now,
the addition of all the miscellaneous bits and bobs to support the main
56K assembler are in place but they don't cause any regressions to the
existing assemblers already present in RMAC. Stay tuned for Round 2!