Expose `AsyncResult` type alias

[tnull]

In #3734 and #3752 we started converting some of our traits to return an AsyncResult.

However, as async_poll.rs is not exposed, the docs now show the full Result type, which is a mouth full, e.g.:

 fn list_confirmed_utxos<'a>(
        &'a self,
    ) -> Pin<Box<dyn Future<Output = Result<Vec<Utxo>, ()>> + Send + 'a>>;

IMO we should expose (and document) the AsyncResult type alias, to allow users to use AsyncResult in their trait implementations, and to have docs read better.

[joostjager]

I wonder though if it is really better to expose that type? At some point we'll get native async trait support?

[tnull]

I wonder though if it is really better to expose that type? At some point we'll get native async trait support?

Right, but until we get that, it might be convenient to have, IMO. FWIW, I ended up needing to copy/paste from the rendered docs, as the trait definition (and the auto-complete of my editor) used the type alias which I couldn't use.