Add replayLatest

[natebosch]

For non-broadcast streams this would implicitly call .asBroadcastStream(). For any new listeners on a Stream the most recently emitted event will always be emitted.

This one might not quite be in the same scope as most stuff in this package since it will require a new Stream implementation that lets us have some behavior for every new listener. I think by default with a broadcast StreamController you only get the onListen happening any time we go from zero to 1 listeners, not when we get new listeners. I also don't think it has the right hook to send an event to just one listener.

Since that's also a significant deviation from normal Stream behavior we should be careful here and avoid it if it would be too confusing.