Add send_ready

[eholk]

This would take a set of channels and send on one of the ready ones.

We don't really have a notion yet of ready channels, but we could probably build a buffered_stream protocol that would include acknowledgments. This would be helpful for a few things, I imagine.

Part of #1255.

[bblum]

Couldn't a user implement this with something like for list.each |chan| { if chan.try_send(msg) { return true; } } return false; ? Like #3018 I don't think this needs to be a primitive.

[bblum]

try_send is a different concept actually; this would have to be a primitive.

[pnkfelix]

Accepting P-low.

[tomjakubowski]

Would this be a function on &[SyncSender<T>]?

[alexcrichton]

Closing.

See this comment: #3020 (comment)