Experiment with unbounded channel types that do less allocation

[brson]

The current implementation of Chan and Port use a very simple strategy based on an atomic linked list. The implementation is alluringly simple but it requires an allocation on every send and benchmark profiles show these allocations at the top.

Let's evaluate other options.

[huonw]

I'd guess that (an atomic version of) extra::ringbuf would be a way to handle this.

[huonw]

Google turns up http://www.cs.tau.ac.il/~adamx/ppopp2013-x86queues.pdf ("Fast Concurrent Queues for x86 Processors" Morrison & Afek 2013), and that seems to suggest there aren't [m]any other array-based unbounded concurrent queues.

[brson]

We could also experiment with the queue added in #9710. It is surely faster than the existing channels.

[alexcrichton]

Closing. I've implemented an unbounded queue for Chan types which amortized doesn't perform allocations. We'd still need to do some work on SharedChan, but it's clear that avoiding allocations is a huge win here.