Support proc-local functions

[RobSpringer]

It'd be nice to have procs be self-contained, as objects are in OOP. IOW, it'd be nice to be able to do:

proc p {
  config(...) { ... }

  next(tok: token, state: state_t) {
    let new_state = create_new_state(...);
  }

  fn create_new_state(...) -> state_t { ... }
}

That way we can "localize" concerns, rather than having to splat them all over and mix them in a impl. file.

[proppy]

related https://github.com/google/xls/commit/1af3c8cf8144f46be9bca6e44b98477adfe5dd8d