Make bound event listeners Copy

[maciejhirsz]

This should now be legal:

bind! {
    state:

    let reset = |_| *state = 0;
}

view! {
    // `reset` previously was of `impl Listener<_>` type that made it `!Copy`
    <input onclick={reset} onchange={reset}>
}

[maciejhirsz]

Once return position impl Trait in traits becomes stable this can become part of the IntoListener<E> trait.