build,pass: configurable code generation

[mmcloughlin]

Consider adding compilation options to control how avo code generation works.

Thus far a few use cases have come up, but there are undoubtedly more.

• Disable any "optimization" passes avo applies, like pruning self moves or redundant jumps build: cli option to disable instruction modification #82
• Control handling of the base pointer register. Specifically, an option to remove it from allocation altogether, or disable automatic saving of the base pointer register with the "frame size hack" all: avoid clobbering BP register #156