This PR introduces a `Context` object that is stored in the `Completion`
that currently only stores a `Waker`. In the future, I want to add some
sort of abort signal so that we can abort tasks that share the same
Context. To pass the Waker, I introduced a `step_with_waker` function in
`Statement` that delegates to an internal `_step` function. `_step` is
the previous `step` but just with the `Option<&Waker>` argument.
I was going to try and have the BusyHandler by truly async as well, but
I decided to not do it here, because it will be slightly complicated to
achieve.
Closes#3535
6fe9ea0925