Add EventHandler trait to replace EventFn

[a1phyr]

Replace EventFntrait with EventHandler.

The new trait has new capabilities but can be implemented for more types, such as channel receivers which are very commonly used.

[0xpr03]

Hi, can you rebase this onto the current master ? Other than that it seems fine, except for more documentation.

[a1phyr]

Done

[0xpr03]

Thanks! I'll add an example to the lib file. Maybe it'll be worth wile to add a tokio feature where we also implement this for async tokio channels. Problem is though, that we'll have to select between tokio::sync::broadcast and tokio::sync::watch or tokio::sync::mpsc.