feat(traits): add marker traits to schema types

Based on their involvement in activities.
It nearly works perfectly.

src/rust/lib.rs

@@ -3,6 +3,26 @@ extern crate hyper;
 extern crate "rustc-serialize" as rustc_serialize;
 extern crate "yup-oauth2" as oauth2;
 
+use std::marker::MarkerTrait;
+
+/// Identifies types which can be inserted and deleted.
+/// Types with this trait are most commonly used by clients of this API.
+pub trait Resource: MarkerTrait {}
+
+/// Identifies types which are used in API responses.
+pub trait ResponseResult: MarkerTrait {}
+
+/// Identifies types which are used in API requests.
+pub trait RequestResult: MarkerTrait {}
+
+/// Identifies types which are only used as part of other types, which 
+/// usually are carrying the `Resource` trait.
+pub trait Part: MarkerTrait {}
+
+/// Identifies types which are only used by other types internally.
+/// They have no special meaning, this trait just marks them for completeness.
+pub trait NestedType: MarkerTrait {}
+
 /// This module is for testing only, its code is used in mako templates
 #[cfg(test)]
 mod dev;