message: Remove `Message: Clone` bound in `Sandbox`
diff: @@ -81,7 +81,7 @@ pub trait Sandbox {
  /// The type of __messages__ your [`Sandbox`] will produce.
  ///
  /// [`Sandbox`]: trait.Sandbox.html
  - type Message: std::fmt::Debug + Send + Clone;
  + type Message: std::fmt::Debug + Send;
  /// Initializes the [`Sandbox`].
  ///
  