message: Update iter_custom to work in 0.3.0
diff: @@ -398,10 +398,10 @@ impl<'a, M: Measurement> Bencher<'a, M> {
  #[inline(never)]
  pub fn iter_custom<R>(&mut self, mut routine: R)
  where
  - R: FnMut(u64) -> Duration,
  + R: FnMut(u64) -> M::Value,
  {
  self.iterated = true;
  - self.elapsed = routine(self.iters);
  + self.value = routine(self.iters);
  }
  #[doc(hidden)]
  