5 backends

[skx]

This pull request, once complete, will significantly overhaul our code:

• Drop most code from main.go and move it into packages.
• Allow compilation of our program via either nasm or gcc
  • By allowing the user to choose a suitable backend

Current status is that assembly generation works as well as it did before, but we don't yet have a working C-backend.

Once complete this will close #5.