std: Change Times trait to use do instead of for

Change the former repetition::

    for 5.times { }

to::

    do 5.times { }

.times() cannot be broken with `break` or `return` anymore; for those
cases, use a numerical range loop instead.

src/libextra/json.rs

@@ -77,7 +77,7 @@ fn escape_str(s: &str) -> ~str {
 
 fn spaces(n: uint) -> ~str {
     let mut ss = ~"";
-    for n.times {
+    do n.times {
         ss.push_str(" ");
     }
     return ss;