collection resolver producer progress

include/matador/object/collection_proxy.hpp

@@ -36,12 +36,17 @@ public:
 template<typename Type>
 class collection_proxy final {
 public:
+  collection_proxy() = default;
+
+  // Lazy
   collection_proxy(std::weak_ptr<object_resolver<typename Type::value_type>> resolver, utils::identifier owner_id)
   : owner_id_(std::move(owner_id)), resolver_(std::move(resolver)) {}
 
+  // Eager
   collection_proxy(std::weak_ptr<object_resolver<typename Type::value_type>> resolver, std::vector<Type> items)
   : items_(std::move(items)), resolver_(std::move(resolver)) {}
 
+  // Transient
   explicit collection_proxy(std::vector<Type> items)
   : items_(std::move(items)) {}