message: Add missing StorageIterator import
diff: use cosmwasm_std::{coins, BankMsg, HumanAddr, InitResult, MigrateResponse, Order, StdError};
  use cosmwasm_vm::testing::{init, migrate, mock_env, mock_instance, MOCK_CONTRACT_ADDR};
  +use cosmwasm_vm::StorageIterator;
  use burner::msg::{InitMsg, MigrateMsg};
  use cosmwasm_vm::{ReadonlyStorage, Storage};
  