more tests of tuples, mixing positional and explicit field names (#480)

* more tests of tuples, especially mixing positional and explicit field names

* test of match with nested tuple

* Update executable_semantics/testdata/fun_named_params2.6c

Co-authored-by: Geoff Romer <gromer@google.com>

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/tuple3.6c

@@ -0,0 +1,8 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> Int {
+  var (.x = Int, Int): t = (3, .x = 2);
+  return t.x + 1 - t[0];
+}