Added support for a \0 escape sequence.

This commit adds support for `\0` escapes in character and string literals.

Since `\0` is equivalent to `\x00`, this is a direct translation to the latter
escape sequence. Future builds will be able to compile using `\0` directly.

Also updated the grammar specification and added a test for NUL characters.

src/test/run-pass/nul-characters.rs

@@ -0,0 +1,44 @@
+// Copyright 2013 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.
+
+pub fn main()
+{
+    let all_nuls1 = "\0\x00\u0000\U00000000";
+    let all_nuls2 = "\U00000000\u0000\x00\0";
+    let all_nuls3 = "\u0000\U00000000\x00\0";
+    let all_nuls4 = "\x00\u0000\0\U00000000";
+
+    // sizes for two should suffice
+    assert_eq!(all_nuls1.len(), 4); 
+    assert_eq!(all_nuls2.len(), 4);
+
+    // string equality should pass between the strings
+    assert_eq!(all_nuls1, all_nuls2);
+    assert_eq!(all_nuls2, all_nuls3);
+    assert_eq!(all_nuls3, all_nuls4);
+    
+    // all extracted characters in all_nuls are equivalent to each other
+    for c1 in all_nuls1.iter()
+    {
+        for c2 in all_nuls1.iter()
+        {
+            assert_eq!(c1,c2);
+        }
+    }
+    
+    // testing equality between explicit character literals
+    assert_eq!('\0', '\x00');
+    assert_eq!('\u0000', '\x00');
+    assert_eq!('\u0000', '\U00000000');
+
+    // NUL characters should make a difference
+    assert!("Hello World" != "Hello \0World");
+    assert!("Hello World" != "Hello World\0");
+}