Improved to the point where it compiles Michael's verticality demo.

Improved to the point where it compiles Michael's verticality demo.

Generalized the way that segments are handled all over the place, making
it easier to move away from a.out if necessary, and cleaner to boot.

Made DC behave approximately as it should, although the new code is ugly
and bugly.

Fixed unary minus.

Made code generation behave much more like devpac by default, which
helps a lot with the TOS PRG fixup problem.

Fixed a whole bunch of instruction encoding errors.