Fix soundness bug with Rt<Option<T>>

Was thinking about the implications of this, and if you ever have a &mut
Option<Rt<T>> you could call `Option::take` and get the `Rt<T>` out. This would
be unsound. Still need to think about how to support optionals in functions that
take `&mut Context`.

src/core/gc/root.rs

@@ -444,19 +444,16 @@ impl<T, U> DerefMut for Rt<(T, U)> {
     }
 }
 
+// Can't implement [`DerefMut`] because it would allow you to call
+// [`Option::take`] which would return an owned Rt and break the chain of
+// traceability
 impl<T> Deref for Rt<Option<T>> {
     type Target = Option<Rt<T>>;
     fn deref(&self) -> &Self::Target {
         unsafe { &*(self as *const Self).cast::<Self::Target>() }
     }
 }
 
-impl<T> DerefMut for Rt<Option<T>> {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        unsafe { &mut *(self as *mut Self).cast::<Self::Target>() }
-    }
-}
-
 impl<T> Rt<Option<T>> {
     pub(crate) fn set<U: IntoRoot<T>>(&mut self, obj: U) {
         unsafe {