Merge step_by and step_until into a unique method

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`).

asynchronix/examples/assembly.rs

@@ -175,7 +175,7 @@ fn main() -> Result<(), SimulationError> {
 
     // Advance simulation time by 0.9s, which with a 10Hz PPS should correspond to
     // 9 position increments.
-    simu.step_by(Duration::new(0, 900_000_000))?;
+    simu.step_until(Duration::new(0, 900_000_000))?;
     t += Duration::new(0, 900_000_000);
     assert_eq!(simu.time(), t);
     for _ in 0..9 {