matrix-sdk-crypto missing facility to signal rotation of a verified cryptographic identity

matrix-rust-sdk is an implementation of a Matrix client-server library in Rust. Versions of the matrix-sdk-crypto Rust crate before 0.8.0 lack a dedicated mechanism to notify that a user's cryptographic identity has changed from a verified to an unverified one, which could cause client applications relying on the SDK to overlook such changes. matrix-sdk-crypto 0.8.0 adds a new VerificationLevel::VerificationViolation enum variant which indicates that a previously verified identity has been changed.