Add `#[deprecated]` attribute

[cdleary]

Deprecation attributes are kind of interesting, because you might think they should flag a warning. But then for anybody who does warnings-as-errors (as we do), deprecating something in a way that causes an error would mean you have to delete it right away!

So you kind of figure deprecation warnings must be more like "notices". However, it seems like, in reality, they only show up in code search indices and maybe language server clients, e.g. as struck out identifiers to make for a helpful indicator.

So there's no deprecation "warnings" in that case, just kind of helpful strikethroughs in IDEs.

In any case, we should be able to mark an item as deprecated, though we then (I expect) will not produce warnings when it is used.