Manifest checks

[epage]

The API extends past the rust code to the manifest

Possible checks include

• Explicit feature removed (major breaking)
• Implicit feature (optional dependency) removed (informative)
  • This is separate because pre-1.60's namespaced features, crates couldn't hide these
• Implicit feature added via removing dep: reference to optional dependency (informative)
• Explicit feature no longer implies another explicit feature (major breaking)
• Feature no longer implies an implicit feature (informative)