bindings: double down on Wakers

[camshaft]

Problem:

None of our poll_* functions actually take a cx: &mut std::task::Context, which is the standardized interface for futures in Rust. Instead, applications are expected to call conn.set_waker(cx.waker()) before calling poll_*.

Solution:

We should, instead, double down on async, and require std::task::Context to be passed to all functions that have the signature fn poll_*(&mut self, cx: &mut std::task::Context) -> Poll<Result<_>>;. For blocking IO applications, it's very trivial to make no-op wakers, especially with the coming Waker::noop method in std (see rust-lang/rust#98286). We can also add blocking APIs that construct noop wakers, if this is too painful as well.

[maddeleine]

We would like this this before moving to 1.0.