syntax: allow stmt/expr macro invocations to be delimited by [].

this is useful for macros like vec! which construct containers

src/test/run-pass/vec-macro-with-brackets.rs

@@ -0,0 +1,23 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(macro_rules)]
+
+macro_rules! vec [
+    ($($e:expr),*) => ({
+        let mut _temp = ::std::vec::Vec::new();
+        $(_temp.push($e);)*
+        _temp
+    })
+]
+
+pub fn main() {
+    let my_vec = vec![1, 2, 3, 4, 5];
+}