This adds the concept of label annotations. The primary driver of
the feature is the desire to note that sometimes you know what a
thing is, but sometimes you're just taking an educated guess.
Instead of writing "high_score_maybe", you can now write "high_score?",
which is more compact and consistent. The annotations are stripped
off when generating source code, making them similar to Notes.
I also created a "Generated" annotation for the labels that are
synthesized by the address table formatter, but don't modify the
label for them, because there's not much need to remind the user
that "T1234" was generated by algorithm.
This also lays some of the groundwork for non-unique labels.
