Avoid writing `config.toml` and other files if no significant changes.

[anforowicz]

After this commit audits.toml, config.toml, and imports.lock will not be written if their contents wouldn't have changed ("contents" in a business logic sense - ignoring comments and/or formatting differences).

Fixes https://github.com/mozilla/cargo-vet/issues/589

[anforowicz]

/cc @danakj

[anforowicz]

Ooops... I neglected to update test expectations... Let me fix this...

[anforowicz]

Ooops... I neglected to update test expectations... Let me fix this...

Done

• unknown_field_config - apparently unknown fields to not interfere with parseability of toml files, so now this succeeds. Being resilient to old or future fields seems desirable in general.
• invalid_formatting - this is what this PR is all about - ignoring inconsequential formatting changes
• project-json and project-suggest-json - AFAICT this just reordered some toml items, but made no "actual" changes. So, this LGTM.
• dump-graph-full and dump-graph-full-json. The changed expectation here LGTM, but I only reviewed them in a somewhat shallow way. Please shout if there is some additional manual verification that I should attempt.

[anforowicz]

Abandoning this PR based on https://github.com/mozilla/cargo-vet/issues/589#issuecomment-1998315628