Add specializations for reference types

[cfrantz]

The serde library provides forwarding implementations of serde::Serialize for &T for all T where T: Serialize.

It also provides forwarding implementations for:

• &mut T
• Box<T>
• Rc<T> and RcWeak<T>
• Arc<T> and ArcWeak<T>
• Cow<'a, T>

Write test cases for each of these and fix serde-annotate appropriately.