The API style is now more uniform: both are passed by mutable ref, and
only expose accessors. Additionally, the methods that were initially
accessed through the scheduler field are now directly implemented on
`Context`.
Now that `step_by` returns an error anyway (it was unfaillible before),
there is no more incentive to keep it as a separate method.
The `step_until` method now accepts an `impl Deadline`, which covers
both cases (`Duration` and `MonotonicTime`).
