Add send_any for pipes

[eholk]

Part of #1255.

This would take a list of chans and a message and send it on one of them.

I'm not entirely sure how this is useful, but we did have a request for it.

[bblum]

compared to #3020, this one is even more obviously user-implementable and doesn't need to be a primitive. I feel like this can be closed.

[bblum]

see discussion in #3020; this is worth leaving open

[thestinger]

Removed by #8170, the bug tracking a rewrite is #7668.

[bblum]

This doesn't have to do with the pipe compiler; it's just a convenience function that could work for new runtime pipes as well.

[pnkfelix]

Accepting P-low.

[tomjakubowski]

I'm not sure what this should do (how the sender is picked): if this function were on &[Sender<T>], it might send to the first of them whose receiver hadn't hung up; if it were on &[SyncSender<T>], then it might send to the first sender that didn't already have a full buffer.

[alexcrichton]

Closing.

See this comment: #3020 (comment)