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/syntax/paren_contents.cpp

@@ -0,0 +1,26 @@
+// 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
+
+#include "executable_semantics/syntax/paren_contents.h"
+
+namespace Carbon {
+
+Expression* ParenContents::AsExpression(int line_number) const {
+  if (fields_.size() == 1 && fields_.front().name == "" &&
+      has_trailing_comma_ == HasTrailingComma::No) {
+    return fields_.front().expression;
+  } else {
+    return AsTuple(line_number);
+  }
+}
+
+Expression* ParenContents::AsTuple(int line_number) const {
+  auto vec = new std::vector<std::pair<std::string, Carbon::Expression*>>();
+  for (const FieldInitializer& initializer : fields_) {
+    vec->push_back({initializer.name, initializer.expression});
+  }
+  return MakeTuple(line_number, vec);
+}
+
+}  // namespace Carbon