DRTVWR-558: Constrain LL::apply()'s use of std::apply().

Once std::apply() becomes available, 'using std::apply;' isn't correct because
the more general template tries to handle the apply(function, vector) case
that we explicitly implement below. Have to provide apply(function, tuple) and
apply(function, array) signatures that can forward to std::apply().

indra/llcommon/apply.h

@@ -93,7 +93,14 @@ auto invoke(Fn&& f, Args&&... args)
 #if __cpp_lib_apply >= 201603L
 
 // C++17 implementation
-using std::apply;
+// We don't just say 'using std::apply;' because that template is too general:
+// it also picks up the apply(function, vector) case, which we want to handle
+// below.
+template <typename CALLABLE, typename... ARGS>
+auto apply(CALLABLE&& func, const std::tuple<ARGS...>& args)
+{
+    return std::apply(std::forward<CALLABLE>(func), args);
+}
 
 #else // C++14
 
@@ -124,6 +131,8 @@ auto apply(CALLABLE&& func, const std::tuple<ARGS...>& args)
                       std::index_sequence_for<ARGS...>{});
 }
 
+#endif // C++14
+
 // per https://stackoverflow.com/a/57510428/5533635
 template <typename CALLABLE, typename T, size_t SIZE>
 auto apply(CALLABLE&& func, const std::array<T, SIZE>& args)
@@ -131,8 +140,6 @@ auto apply(CALLABLE&& func, const std::array<T, SIZE>& args)
     return apply(std::forward<CALLABLE>(func), std::tuple_cat(args));
 }
 
-#endif // C++14
-
 /*****************************************************************************
 *   bind_front()
 *****************************************************************************/