expose listener count

[beckend]

Using this as a base to create libraries, it's very useful to know the listener count from time to time.

[notgull]

This can be done with relative ease on std platforms, but on no_std it's impossible to know how many listeners are in the last at any given time.

[beckend]

Sure, then a feature flag?

[notgull]

It sounds fine to restrict it to std only. I'll accept a PR

[beckend]

PR is online