C arrays are iterable too!

(This doesn't really *do* anything, since we have an overload for printing C
arrays to take precedence over array->pointer decay, but it's more correct, and
that makes it more good.)

include/mettle/output/traits.hpp

@@ -38,7 +38,7 @@ namespace mettle {
 
   template<typename T>
   struct is_iterable<T, std::void_t<
-    decltype(std::begin(std::declval<T>()), std::end(std::declval<T>()))
+    decltype(std::begin(std::declval<T&>()), std::end(std::declval<T&>()))
   >> : std::true_type {};