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/librustc/middle/cfg/construct.rs

@@ -467,7 +467,8 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             ast::ExprCast(e, _) |
             ast::ExprUnary(_, e) |
             ast::ExprParen(e) |
-            ast::ExprField(e, _, _) => {
+            ast::ExprField(e, _, _) |
+            ast::ExprTupField(e, _, _) => {
                 self.straightline(expr, pred, [e])
             }