libSyntax: Support tuple expression.

Tuple expression essentially has the same underlying structure as
function call arguments in libSyntax. However, we separate them as
different libSyntax kinds for better usability.

Different from AST, libSyntax currently allows single-child,
label-free tuple expressions (represented as ParenExpr in AST). This is
subject to change if we need to adopt the same differentiation in
libSyntax in the future.

utils/gyb_syntax_support/ExprNodes.py

@@ -19,6 +19,9 @@ EXPR_NODES = [
     Node('FunctionCallArgumentList', kind='SyntaxCollection',
          element='FunctionCallArgument'),
 
+    Node('TupleElementList', kind='SyntaxCollection',
+         element='TupleElement'),
+
     Node('ArrayElementList', kind='SyntaxCollection',
          element='ArrayElement'),
 
@@ -131,6 +134,13 @@ EXPR_NODES = [
              Child('RightParen', kind='RightParenToken'),
          ]),
 
+    Node('TupleExpr', kind='Expr',
+         children=[
+             Child('LeftParen', kind='LeftParenToken'),
+             Child('ElementList', kind='TupleElementList'),
+             Child('RightParen', kind='RightParenToken'),
+         ]),
+
     # Array literal, e.g. [1, 2, 3]
     Node('ArrayExpr', kind='Expr',
          children=[
@@ -159,6 +169,18 @@ EXPR_NODES = [
                    is_optional=True),
          ]),
 
+    # An element inside a tuple element list
+    Node('TupleElement', kind='Syntax',
+         children=[
+             Child('Label', kind='IdentifierToken',
+                   is_optional=True),
+             Child('Colon', kind='ColonToken',
+                   is_optional=True),
+             Child('Expression', kind='Expr'),
+             Child('TrailingComma', kind='CommaToken',
+                   is_optional=True),
+         ]),
+
     # element inside an array expression: expression ','?
     Node('ArrayElement', kind='Syntax',
          children=[