Clippy configuration avoid-breaking-public-api

[BartMassey]

Clippy has a "configuration variable" avoid-breaking-exported-api which is on by default. It inhibits a whole bunch of useful Clippy lints for the signatures of externally-visible items, on the grounds that changing them would be a semver breaking change. (It is also applied to binary crate items, which is almost certainly a mistake: I have filed an issue.)

I think we could use a guideline that suggests adding a clippy.toml to the project with

 avoid-breaking-exported-api = false

in it when creating the project, then setting it to true after the API is stabilized, and then temporarily setting it to false after making other breaking changes so that potential API-breaking lints can be reviewed and considered.

(I don't like this approach: I will personally be permanently setting this variable to false in my projects, and monitoring API breaks "manually". I would strongly prefer a command-line argument to Clippy to set configuration variables, and would strongly prefer that this one default to false. That said, it is what it is — that's a discussion for another repo.)

I think most folks aren't even aware that Clippy's configuration mechanism even exists, much less that it is configured to ignore important API infelicities by default. I wasn't.

I'd be happy to put in a PR for this (with neutral language, of course) if folks think this is a good idea.