Add builtin for lvalue_or_rvalue helper

rt/include/python/builtins.hpp

@@ -237,4 +237,27 @@ typon::Task<PyFile> open(const PyStr &path, std::string_view mode) {
 
 #include "../typon/generator.hpp"
 
+#include <pybind11/embed.h>
+#include <pybind11/stl.h>
+namespace py = pybind11;
+
+#include <utility>
+
+template <typename Ref>
+struct lvalue_or_rvalue {
+
+  Ref &&ref;
+
+  template <typename Arg>
+  constexpr lvalue_or_rvalue(Arg &&arg) noexcept
+      :   ref(std::move(arg))
+  { }
+
+  constexpr operator Ref& () const & noexcept { return ref; }
+  constexpr operator Ref&& () const && noexcept { return std::move(ref); }
+  constexpr Ref& operator*() const noexcept { return ref; }
+  constexpr Ref* operator->() const noexcept { return &ref; }
+
+};
+
 #endif // TYPON_BUILTINS_HPP