refactor(PR #11): rework Merkle tree layer and error type, remove sled and pmtree utilities, migrate to pmtree v3, fix override_range bug, generic PmTree<D,H> struct for any persistent backend

refactor(PR #11): rework Merkle tree layer and error type, remove sled and pmtree utilities, migrate to pmtree v3, fix override_range bug, generic PmTree<D,H> struct for any persistent backend #1911