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