[patch] Some std::locale improvements

[Jonathan Wakely]

Finally, does anyone know why we don't use the stored size in the
facet virtual functions that return strings? e.g.

      virtual string
      do_grouping() const
      { return _M_data->_M_grouping; }

We could save the cost of a strlen call:

      virtual string
      do_grouping() const
      { return string(_M_data->_M_grouping, _M_data->_M_grouping_size); }