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`).
The build context is now passed as a mutable reference due to the need
to mutate data when adding a model.
Contains small unrelated cleanups and documentation improvements too.
