[feat] kv_typed: add backend memory_singleton

rust-libs/tools/kv_typed/src/as_bytes.rs

rust-libs/tools/kv_typed/src/backend.rs

rust-libs/tools/kv_typed/src/backend/memory_singleton.rs

rust-libs/tools/kv_typed/src/explorer.rs

rust-libs/tools/kv_typed/src/from_bytes.rs

rust-libs/tools/kv_typed/src/key.rs

rust-libs/tools/kv_typed/src/lib.rs