Refactor serde and ScalarValue relations

[tyranron]

Required for #975

Synopsis

This PR refactors the way serde is paired with ScalarValues. Previously, ScalarValue exposed Visitor associated type for InputValue<T> being able to delegate scalar value deserialization, #[derive(GraphQLScalarValue)] was always providing a Serializeimplementation. Such approach was causing issues for implementing ScalarValue for foreign types, because in most de::Visitor types are private and not exposed, which makes us to duplicate deserialization logic in our code, and user looses control over serialization.

Solution

The refactored approach requires only T: Deserialize for impl Deserialize for InputValue<T>, and, additionally, removes impl Serialize from the code generated by #[derive(GraphQLScalarValue)]. This allows:

• to reuse any existing Deserialize impls for foreign types and require to do squats with exposing Visitor;
• to give user full control over type serialization/deserialization.

Checklist

• tests updated
• documentation updated
• changelog updated