Avoid polling when waiting on channels

[daviddrysdale]

Possibly based on absl::Notification (although that says multiple calls to Notify() are undefined behaviour, so it's not an immediate fit).

[blaxill]

The rust runtime also currently uses sleep. We should instead implement correct thread suspension. This could possibly be based on Condvar in rust or through some use of the future framework.

[blaxill]

Some notes

• crossbeam appears to spin for no_std link

• crossbeam wait_group example - a simple example of multiple threads waiting on a single Condvar

• The wait_on_channels case is the most difficult as a single Condvar isn't enough.

• Futures with no_std support is mixed as any actual implementations need OS support for thread-things.

• Assuming we go with an enclave abstraction layer, this PR will likely use sync primitives defined there

[daviddrysdale]

For the C++ side of this, I've dumped my experiments into #589.

Also, there's some interesting and possibly relevant stuff in "Go channels on steroids"