Function instantiate 

pub fn instantiate<A, S, Q, M, U>(
    instance: &mut Instance<A, S, Q>,
    env: Env,
    info: MessageInfo,
    msg: M,
) -> ContractResult<Response<U>>
where
    A: BackendApi + 'static,
    S: Storage + 'static,
    Q: Querier + 'static,
    M: Serialize,
    U: DeserializeOwned + CustomMsg,

Mimics the call signature of the smart contracts. Thus it moves env and msg rather than take them as reference. This is inefficient here, but only used in test code.