Add support for preprocessed JSON strings (with optimizations) in encoder

In some situations, you may have a large python dictionary you need to JSONify but one of the values inside the dict is already a JSON string. This is common when pulling an object from a database, for example, where one of the fields is a JSON blob/string. Previously you would have to deserialize and then reserialize that string just to serialize the high level object, but obviously this is unnecessarily slow. This changes adds a method/type that can be used to wrap a str and tell the serializer to just pass it through instead.