Add support for returning a pre-existing foreign object from a foreign method

This is potentially subject to change in the near
future. This commit adds support for returning
an instance (currently wrapped in ForeignObject<>)
of a foreign object that is already instantiated.
The object returned this way simply gets selected
by handle into slot 0 of C Wren.
Conversely, returning just a naked object causes
wrenpp to make a Wren-accessible copy of it.

examples/calendar/main.cpp

@@ -128,7 +128,7 @@ int main() {
 
 	vm.interpret("main", g_test_script);
 
-	Calendar* const cale = std::get<0>(wren::variables<Calendar>(vm, MN<"main", "cale">));
+	Calendar* const cale = std::get<0>(wren::variables<Calendar>(vm, MN<"main", "cale">)).data();
 	cale->print_appointments();
 
 	return 0;