Add types module

Move `CompositeOperator` and `ResourceType` to the `types` module

src/types/mod.rs

@@ -0,0 +1,7 @@
+mod composite_operator;
+mod metric_type;
+mod resource_type;
+
+pub use self::composite_operator::CompositeOperator;
+pub use self::metric_type::MetricType;
+pub use self::resource_type::ResourceType;