fix bounds checking failure message

casting the `uint` to an `int` can result in printing high values as
negative intege

src/libstd/unstable/lang.rs

@@ -25,7 +25,7 @@ pub fn fail_(expr: *c_char, file: *c_char, line: size_t) -> ! {
 pub fn fail_bounds_check(file: *c_char, line: size_t,
                          index: size_t, len: size_t) {
     let msg = format!("index out of bounds: the len is {} but the index is {}",
-                      len as int, index as int);
+                      len as uint, index as uint);
     do msg.with_c_str |buf| {
         fail_(buf, file, line);
     }