Make user types default constructible (for now)

trans/transpiler/phases/emit_cpp/class_.py

@@ -21,6 +21,7 @@ class ClassVisitor(NodeVisitor):
         yield "template<typename... T> type(T&&... args) {"
         yield "__init__(std::forward<T>(args)...);"
         yield "}"
+        yield "type() {}"
         yield "type(const type&) = delete;"
         yield "type(type&&) = delete;"
         yield "};"