refactor lint mechanism to support "error in new edition, lint in older editions" pattern

[nikomatsakis]

We've been adding lints that, in newer editions, become hard errors. Right now we do this by manually creating either an error or a lint. (see e.g. #83213). We should introduce an API where we write one specification and the compiler decides, based on the edition of a particular span, whether to introduce a hard error or not.

[nikomatsakis]

I've marked this as E-mentor. I'm game to help someone with it!

Some mentoring notes:

• rustc-dev-guide section on errors
• Check out Update BARE_TRAIT_OBJECT and ELLIPSIS_INCLUSIVE_RANGE_PATTERNS to errors in Rust 2021 #83213 to see some examples of how the code to be cleaned up looks.

[petrochenkov]

BARE_TRAIT_OBJECT is not a good case for using such mechanism - #83213 (comment).

[nikomatsakis]

OK; the mechanism may not be used enough to warrant existing =)

[nikomatsakis]

After some discussion, decided this is premature.