Replace try_send/try_recv with is_closed

[brson]

These methods combine two operations: send/recv and check whether the channel is closed. try_recv in particular does not do what you would expect - recv or return immediately if no message is available.

No expressiveness would be lost by making the is_closed check its own method, though it may be slightly less efficient.

Nominating.

[brson]

Oh, actually removing try_recv completely means you can't block waiting for a message, then be woken up when the other end closes without sending. There does need to continue to be a try_recv equivalent. It just needs a better name.

[brson]

Maybe rename try_recv to recv_opt.

[brson]

try_recv may be an antipattern without actual users. We could try just getting rid of it.

[jdm]

We use it an a number of locations in Servo, fwiw.

[TeXitoi]

I use it for a receiver collecting every messages send by tasks via SharedChan (ie map reduce).

Why not replace actual try_recv() by an implémentation of Iterator::next()?

[nikomatsakis]

It seems like moving to is_closed would introduce a race.

[pnkfelix]

delegating to @alexcrichton to clean tags post nomination.

[alexcrichton]

Closing in favor of #10459.