Add IntoEvent macro which automatically implement Into<Event> to a struct

[loloicci]

This PR suggests and implements a derive macro for structs named IntoEvent which is for developers of contracts.
With this, #[derive(IntoEvent)] implements Into<Event> to a decorated struct.
This makes it easier to make a structure representing an event.

An Example usage:

#[derive(Clone, IntoEvent)]
struct TransferEvent {
    from: Addr,
    receiver: Addr,
    #[to_string_fn(coins_to_string)]
    amount: Vec<Coin>,
}

let transfer_event = TransferEvent {
    from: Addr::unchecked("alice"),
    receiver: Addr::unchecked("bob"),
    amount: coins(42, "link"),
};

let response = Response::<Empty>::new().add_event(transfer_event);

See the document of derive_into_event in packages/derive/src/lib.rs to learn how to use it.

See tests in packages/std/src/results/response.rs for more example usages.

[chipshort]

Thanks for bringing this up!
I think this is a good approach, but the derive macro is a bit out of scope for cosmwasm-std.
I suggest we merge the Into<Event> change and then the derive macro can be a separate crate that you could even maintain yourself.
Also note that attributes can be added both to events and responses directly, so maybe a more general solution for generating attributes makes more sense? This was brought up in #1928 before.

[chipshort]

Neat!

[loloicci]

@chipshort

I suggest we merge the Into change and then the derive macro can be a separate crate that you could even maintain yourself.

It looks good! Should I reopen a new PR for it? Or will you do it?

[chipshort]

@loloicci Yes, either a new PR or rebase this one. Also, please add a changelog entry for it under the "Unreleased" section.

[loloicci]

@chipshort
OK, I will make a new PR. Thanks.