There are many small ways to break certain semver guarantees, and it would be nice if we could avoid making these mistakes; so let's add some tests to ensure that!

Add tests for:

• Presence (or absence) of marker traits on types:
  • Send
  • Sync
  • UnwindSafe
  • RefUnwindSafe
  • Unpin
• Contains interior mutability (e.g. disallowing placing something in a static)
• Object safety on the traits we expose.
• Variance of any lifetimes and/or generics we expose.

Possibly also: Integrate cargo-semver-checks in CI?