message: split_at_mut in CallStack::trace
  Summary: Minor code cleanup.
diff: @@ -138,12 +138,13 @@ const MAX_CALLSTACK_RECURSION: usize = 40;
  unsafe impl<'v> Trace<'v> for CallStack<'v> {
  fn trace(&mut self, tracer: &Tracer<'v>) {
  - for x in self.stack[0..self.count].iter_mut() {
  + let (used, unused) = self.stack.split_at_mut(self.count);
  + for x in used {
  x.function.trace(tracer);
  }
  // Not required, but since we are chosing not to walk those above
  // the current stack depth, it's good practice to blank those values out
  - for x in self.stack[self.count..].iter_mut() {
  + for x in unused {
  x.function = Value::new_none();
  x.span = None;
  }
  