add ffi stubs for iterators and batch writes
add ffi stubs for iterators and batch writes

kaedroho/rust-rocksdb,tempbottle/rust-rocksdb,zhangjinpeng1987/rust-rocksdb,spacejam/rust-rocksdb,zhangjinpeng1987/rust-rocksdb,tempbottle/rust-rocksdb,kaedroho/rust-rocksdb,dgrnbrg/rust-rocksdb,spacejam/rust-rocksdb,kaedroho/rust-rocksdb,Lingling7/rust-rocksdb,tempbottle/rust-rocksdb,spacejam/rust-rocksdb