Restructure handling of paren expressions (#417)

* Move nontrivial logic out of `parser.ypp` into `FieldList`, rename it to `ParenContents`, make it a class, and add tests
* Use a `FieldInitializer` struct instead of `std::pair<std::string, Expression*>` to represent the fields of a tuple

executable_semantics/testdata/tuple2.6c

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 fn main() -> Int {
-  var (Int,): t = (5,);
-  return t[0] - 5;
+  var (Int,): t1 = (5,);
+  var (Int, Int): t2 = (2, 3,);
+  return t1[0] - t2[0] - t2[1];
 }