Support proc-local functions

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.