Add Channel#trySend

[armanbilge]

Opening to get feedback on the idea, which is to provide a "view" to a Channel as a QueueSink. This allows APIs to use the QueueSink interface and for a Channel to be plugged in as an implementation detail.

To adapt the Channel interface for QueueSink I added:

1. an implementation of trySend (analog to tryOffer), which is a non-blocking version of send.
2. a Channel.ClosedException which is thrown when encountering a Channel.Closed.

Thanks for your consideration.

[armanbilge]

Alternatively, Channel itself could extend QueueSink?

[armanbilge]

Some feedback from @SystemFw

I'm 50/50 about it
Channel has a few semantic aspects that I think you ought to keep in mind when using it: graceful closure, single consumer, chunking by default (most cases of a queue consumed by a stream fall into this), so I'm not too sure about hiding those

I think those are fair points. WDYT about reducing the scope of this PR to just the trySend method? Then a QueueSInk adapter can be a user-land thing, if at all.

[armanbilge]

Ok, I came to appreciate Fabio's point, and I changed my mind about the QueueSink thing. I'd still like the trySend op actually, so I kept that.