Add is_closed and has_message to the Receiver

[faern]

Allows polling the receiver for the channel state, without changing it or without pulling any message out.

Does this allow using oneshot in the way you needed it @szostid ? Sure, if your code needs to know both whether or not the channel is disconnected and if there is a message, this solution requires two method calls (and two atomic loads) instead of one. But is that a real world use case? I imagine for the majority of places where you need to check the state you are either interested in knowing if there is a message to be read, or if the sender gave up and aborted.

I could also consider going more towards your suggested route in #52. But only if we slim down the state enum to just expose Empty, Message, and Disconnected. The three states that any receiver code could reasonably care about? But that method would then not be mutually exclusive with these helper methods. For those that only need to check if a message is available or not, calling has_message is going to provide better API ergonomics than matching an enum.

[szostid]

Yes, this should cover all my use cases. Thanks!

[faern]

I just published version 0.1.10 with this functionality! Please let me know if it does not work as intended, or does not allow the desired use case.