ref(kv_typed): get_ref_slice: deser must be backend agnostic

tools/kv_typed/src/backend.rs

tools/kv_typed/src/backend/leveldb.rs

tools/kv_typed/src/backend/lmdb.rs

tools/kv_typed/src/backend/memory.rs

tools/kv_typed/src/backend/memory_singleton.rs

tools/kv_typed/src/backend/sled.rs

tools/kv_typed/src/collection_ro.rs

tools/kv_typed/src/transactional_read.rs

tools/kv_typed/src/transactional_write.rs