Events: Consider returning a `EventHandlerRegister` from `StdLibrary::handlers`

With the introduction of new the event handling infrastructure, users of `miden-vm` who implement their own `Host` need to manually handle these events. The straight forward approach is to define an `EventHandlerRegistry` for all library-defined events, which would be invoked as a fall-through if none of the native events match the one on the stack.

To make things easier, we make it a convention that libraries return an `EventHandlerRegister` directly which is easier to embed inside an existing `Host` implementation. The main drawback would be that we would then have multiple successive registries, but would make it simpler to include handlers from multiple registries.

As a bonus feature, we should also implement `EventHandler for EventHandlerRegistry` since a collection of handlers is itself a handler.