semver compat check

[gnunicorn]

additional "auto" mode for version-bumping that:

• starting with a set of files (e.g. that changed in a github PR)
• figure out deepest-dependency-first whether the version needs to be bumped, compared to what is released and bump accordingly (using semverver)

Note:

• a major change in a dependency doesn't necessarily means more than a minor is needed for the dependent crate
• pushing through major changes on types that are re-exported must be observed correctly
• semverver is a nightly-only feature
• this has to be feature gated and will only available on nightly for now

[drahnr]

A notable entry to figure the versions out, might be to build upon https://github.com/rust-lang/rust-semverver