message: Add a Trace instance for RefCell
  Summary: Useful if we put RefCell's in our values.
diff: @@ -40,6 +40,7 @@ use crate::{
  };
  use gazebo::any::AnyLifetime;
  use std::{
  + cell::RefCell,
  cmp::Ordering,
  fmt::{Debug, Write},
  };
  @@ -117,6 +118,12 @@ unsafe impl<'v, T: Trace<'v>> Trace<'v> for Option<T> {
  }
  }
  +unsafe impl<'v, T: Trace<'v>> Trace<'v> for RefCell<T> {
  + fn trace(&mut self, tracer: &Tracer<'v>) {
  + self.get_mut().trace(tracer)
  + }
  +}
  +
  unsafe impl<'v, T1: Trace<'v>, T2: Trace<'v>> Trace<'v> for (T1, T2) {
  fn trace(&mut self, tracer: &Tracer<'v>) {
  self.0.trace(tracer);
  