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/libstd/unstable/extfmt.rs

@@ -636,7 +636,7 @@ pub mod rt {
                 buf.push_char(c);
             }
             buf.push_str(s);
-            for diff.times {
+            do diff.times {
                 buf.push_char(padchar);
             }
             return;

src/libstd/unstable/sync.rs

@@ -626,7 +626,7 @@ mod tests {
             let x = Exclusive::new(~~"hello");
             let x2 = x.clone();
             do task::spawn {
-                for 10.times { task::yield(); } // try to let the unwrapper go
+                do 10.times { task::yield(); } // try to let the unwrapper go
                 fail!(); // punt it awake from its deadlock
             }
             let _z = x.unwrap();