Allow taking Fn(EVENT) for callbacks too

[ranile]

Description

This PR introduces a new trait, IntoEventCallback, to allow Fn(EVENT) to be passed for listeners, in addition to Callback.
This is workaround for specialization (or lack thereof) and should be removed once specialization hits.

Fixes #1657 (the main problem described)

Checklist

• I have run cargo make pr-flow
• I have reviewed my own code
• I have added tests

[ranile]

I'm not sure where the tests for this should go. This isn't macro related so yew-macro sounds like the wrong place but there's no where else that runs compile tests.

[mc1098]

Nice workaround :)

How about something like this in that mod.rs file?

#[test]
fn supported_into_event_callback_types() {
    let f = |_: usize| ();
    let cb = Callback::from(f);

    // Callbacks
    let _: Option<Callback<usize>> = cb.clone().into_event_callback();
    let _: Option<Callback<usize>> = (&cb).into_event_callback();
    let _: Option<Callback<usize>> = Some(cb).into_event_callback();

    // Fns
    let _: Option<Callback<usize>> = f.into_event_callback();
    let _: Option<Callback<usize>> = Some(f).into_event_callback();
}

Proves the supported types compile and the test passes :)
I think there are other tests, like the one you fixed, that cover the user/html! side of checking for incorrect EVENT type and adding Callbacks to elements correctly.

[mc1098]

Looks good to me :)