Macros/functions/templates

[tcr]

Codegen is a useful feature in any language, and also an obvious footgun.

1. Extract macros into a language-level feature that allows templates to intersperse AST, FSM transitions, data, and more into code. This is the most flexible and hardest to prototype.
2. Allow us to yield to a function, where the function executes and can terminate, but is expanded into FSM generation. This is a more structured approach to the above, but fits a narrow usecase.
3. Require macros be expanded into their own entities and interfaced with. This probably doesn't fit all usecases either.