At the moment, the user can subscribe to incoming messages by passing a MessageHandler, a default implementation for static Functions is provided, and the user can implement the trait if (s)he needs to.

It could be a good idea to grasp inspirations from other crates to know how such a pattern is handled. Giving the user an infinite iterator over messages looks (naively) like a possibility, but how do such "infinite, blocking iterators" work?

References:

1. Streaming iterator
   May be an interesting approach:

while let Some(message) = eb.consumer("some-address") {
    // this is an infinite loop, though :(
}

2. "Go full async"
   https://github.com/rust-lang/rfcs/blob/master/text/2033-experimental-coroutines.md