Tuple restrictions (#486)

* implement tuple restrictions: (1) named fields after positional, (2) order matters

* after_named_member => seen_named_member

executable_semantics/testdata/record1.6c

@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 fn main() -> Int {
-  var (.x = Int, .y = Int): t2 = (.y = 5, .x = 2);
+  var (.x = Int, .y = Int): t2 = (.x = 2, .y = 5);
   t2.y = 3;
   return t2.y - t2.x - 1; // 3 - 2 - 1
 }