Add a compatibility checker

[jspahrsummers]

Given a user-provided hook which yields arbitrary "compatibility information" (e.g., a list of symbols and associated types) about a project at a specific version, we could potentially build a compatibility checker which end-users could run when trying to determine whether to upgrade.

This could be used to catch SemVer violations, but it could also identify cases where breaking changes were introduced but they don't affect the dependent project in question.

h/t @cbarrett