feat(core): `BlockValidationState` should implement `Debug`

[alexanderwiederin]

BlockValidationState currently does not implement Debug.

Something like the following would make development easier:

rustimpl fmt::Debug for BlockValidationState {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("BlockValidationState")
            .field("mode", &self.mode())
            .field("result", &self.result())
            .finish()
    }
}

ProcessBlockHeaderResult and CheckBlockResult (if merged in #177), should then also implement Debug.