lex() returning NULL is used to convey an error case but having not allocated
any tokens in the case of an empty input file isn't an error case. This patch
causes lex to treat an empty token stream after successfully examining a file
as just a single EOL token. This is a fair approximation of an empty file for
this assembler's purposes, and results in the correct behaviour of an empty
output file.

This emulator provides a rough way for binaries designed for this CPU to be
executed in a virtual/emulated CPU for testing purposes. This patch also adds
a small test setup for automated assembly, execution and checking of register
postconditions for programs.

This somewhat complicates the previous file dissassembly logic, but paves
the way for disassembling instructions from buffered memory. This will be
required for emulation.

Also adds some test cases which nabbed bugs during the development and
testing of this change.

This refactors the disassembler into two stages with the list of struct
instruction (currently also output by the parse stage) as an "intermediate
language" between disassembly and assembler output. This should make these
units, especially the "machine code => IL" section, more reusable for future
soft emulation work.

This allows for inspection of test failures when the script is run with noclean
and the failing test is not the last test in the run.