Helper to forward Deserializer methods to Deserializer::deserialize.
Every given method ignores all arguments and forwards to deserialize.
Note that deserialize_enum simply returns an Error::invalid_type; a
better approach is tracked in serde-rs/serde#521.
Deserializer is a trait that can deserialize values by threading a Visitor trait through a
value. It supports two entry point styles which enables different kinds of deserialization.