Implement tuple and tuple struct indexing

This allows code to access the fields of tuples and tuple structs:

    let x = (1i, 2i);
    assert_eq!(x.1, 2);

    struct Point(int, int);
    let origin = Point(0, 0);
    assert_eq!(origin.0, 0);
    assert_eq!(origin.1, 0);

src/libsyntax/ast.rs

@@ -540,6 +540,7 @@ pub enum Expr_ {
     ExprAssign(Gc<Expr>, Gc<Expr>),
     ExprAssignOp(BinOp, Gc<Expr>, Gc<Expr>),
     ExprField(Gc<Expr>, SpannedIdent, Vec<P<Ty>>),
+    ExprTupField(Gc<Expr>, Spanned<uint>, Vec<P<Ty>>),
     ExprIndex(Gc<Expr>, Gc<Expr>),
 
     /// Variable reference, possibly containing `::` and/or