Add Serialize and Deserialize implementations to Value

[ecton]

Right now Value can't be embedded in a type that derives serialize/deserialize because it doesn't implement those traits. There are two approaches: derive or try to do something special. The derive will clearly work, but when embedding within Rsn it might not render as cleanly as one might hope.

There are a few challenges when trying to do something special:

• Value::Byte is indistinguishable from small unsigned integers stored in a Value::Integer
• Value::Named uses a Map to store its struct contents. Maps allow any key type. Serde requires that struct field names are strings. We could add a new type for the struct contents that enforces the string key requirement to work around this.