Add `unstable` warnings

It would probably be useful to tag certain features as unstable and emit warnings when they're used. I think this will help us more easily get to an MVP, as it lets us separate reliable features from features that are new, prone to change, and less vetted.

The issue proposes tagging certain features as unstable and emitting warnings when they are used, but it names no files, tests, or entry points. First clarify which features should be tagged, how warnings should be represented, and what constitutes an MVP before locating the relevant compiler paths and defining tests for the warning behavior.

Written by the indexing model from the issue text.