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/str.rs

@@ -1828,7 +1828,7 @@ impl<'self> StrSlice<'self> for &'self str {
                 do ret.as_mut_buf |rbuf, _len| {
                     let mut rbuf = rbuf;
 
-                    for nn.times {
+                    do nn.times {
                         ptr::copy_memory(rbuf, buf, len);
                         rbuf = rbuf.offset(len as int);
                     }