Introduce `Word` wrapper for storage map keys

[PhilippGackstatter]

Storage map keys are internally hashed (#1129) and so in storage map related APIs we sometimes deal with the unhashed key and sometimes with the hashed key.

Once we have 0xMiden/crypto#430 we should consider adding wrappers for StorageMapKey and HashedStorageMapKey so the APIs would more clearly communicate what kind of key they expect.

Additionally, some methods could be moved to the key, e.g.:

• StorageMap::hash_key could more naturally be StorageMapKey::hash(&self) -> HashedStorageMapKey.
• StorageMap::hashed_map_key_to_leaf_index (if we still have it) could be moved to HashedStorageMapKey. The absence of that method on StorageMapKey would prevent misusing that key as a LeafIndex.