Add tastyreflect comment API

[abgruszecki]

Exactly what it says in the title.

[abgruszecki]

@nicolasstucki compared to the version from yesterday, there's no more MaybeComment. We assume that comments are always available and fail with an exception if they are not. ReadComments is not always necessary to have comments available - it is, in fact, off in run/tasty-comment test, where we try to get comments from an inline method. I'm pretty sure that if we tried to do that with symbols loaded from a tasty file, the comments would not be available, so there are clearly some inconsistencies. That being said, my student is blocked on not having this API, so I'd strongly prefer to merge this as-is and refine later.