Annotate async_generator, generator, and task with [[nodiscard]]

include/cppcoro/async_generator.hpp

@@ -346,7 +346,7 @@ namespace cppcoro
 	}
 
 	template<typename T>
-	class async_generator
+	class [[nodiscard]] async_generator
 	{
 	public:
 

include/cppcoro/generator.hpp

@@ -160,7 +160,7 @@ namespace cppcoro
 	}
 
 	template<typename T>
-	class generator
+	class [[nodiscard]] generator
 	{
 	public:
 

include/cppcoro/task.hpp

@@ -280,7 +280,7 @@ namespace cppcoro
 	/// caller. Execution of the coroutine body does not start until the
 	/// coroutine is first co_await'ed.
 	template<typename T = void>
-	class task
+	class [[nodiscard]] task
 	{
 	public: