Add trigger_map to support triggering mapped events

[cart]

Objective

Alternative to the suggestion in #14649

Sometimes people want observers to watch for multiple events instead of just one. We should provide some mechanism for this to happen.

Solution

Add a trigger_map function, which returns an ObserverSystem that will use a given map function to trigger a mapped event when a certain event type is triggered:

#[derive(Event, Clone, Debug)]
struct Foo(usize);

#[derive(Event, Debug)]
struct Bar(bool);

#[derive(Event, Debug)]
enum Combined {
    Foo(Foo),
    Bar(Bar),
}

app
    .observe(trigger_map(Combined::Foo))
    .observe(|trigger: Trigger<Combined>| {
        println!("Combined Event: {:?}", trigger.event());
    });

[alice-i-cecile]

A very elegant solution for Or-style logic. I've left a small docs suggestion that I think will help clarify the usage.

[ItsDoot]

Would this work well with bubbling since it's being converted into a different event type? I'm imaging a scenario where someone could end up with this in a hierarchy:

[Observer<Foo>] ---> [Observer<Combined>] ---> [Observer<Foo>]

[ItsDoot]

Alternative here: #14674

[cart]

Closing this for now as I'm not currently thinking about it