LMD-GHOST fork choice integration (#38)
* Move DigestItem to a separate mod * CompatibleExtrinsic trait * Expose API for validator id from attestation * CompatibleExtrinsic impl * Save and load latest attestations * note_block for LatestAttestation * Expose last finalized/justified slot from runtime * Implement LMD-GHOST's is_new_best * Update substrate reference * Finish LMD-GHOST integration * Remove old unused lmd-ghost crate * README: LMD-GHOST is integrated