Add MessageResponse derive

[samrg472]

Related issue #13

[DoumanAsh]

Just to be sure, did you test that derived code gets correctly compiled?

[samrg472]

I tested it against my own codebase locally by removing the manual implementations and replacing it with the derive. Both of the tests below compile successfully.

Test 1

#[derive(Clone, Default, Debug, actix::Message, MessageResponse)]
pub struct MyStruct;

Expands to

mod _impl_act_msg_rsp_mystruct {
    use super::*;
    extern crate actix;
    impl<_A: actix::Actor, _M: Message<Result = MyStruct>>
        actix::dev::MessageResponse<_A, _M> for MyStruct
    {
        fn handle<R: actix::dev::ResponseChannel<_M>>(self, _: &mut _A::Context, tx: Option<R>) {
            if let Some(tx) = tx {
                tx.send(self);
            }
        }
    }
}

Test 2

#[derive(Clone, Default, Debug, Message, MessageResponse)]
pub struct MyStruct<T: 'static> {
    _t: std::marker::PhantomData<T>
}

Expands to

mod _impl_act_msg_rsp_mystruct {
    use super::*;
    extern crate actix;
    impl<T: 'static, _A: actix::Actor, _M: actix::Message<Result = MyStruct<T>>>
        actix::dev::MessageResponse<_A, _M> for MyStruct<T>
    {
        fn handle<R: actix::dev::ResponseChannel<_M>>(self, _: &mut _A::Context, tx: Option<R>) {
            if let Some(tx) = tx {
                tx.send(self);
            }
        }
    }
}

[fafhrd91]

@samrg472 could you create pr with this tests for actix repo. it has tests/derive.rs

[fafhrd91]

@samrg472 could you add entry to changes.md

@DoumanAsh could you make new release