debug mode performance patch

[Paolo Carlini]

On 11/04/2010 09:27 PM, François Dumont wrote:
> Moreover I forget to signal that in _Safe_sequence_base::_M_swap I had
> to lock both sequences mutex. So I hope that the scope_lock are
> reentrant cause for the moment both sequences might be associated to
> the same mutex instance. I hope I haven't been simply lucky during tests.
Before we go ahead with the patch could you please explain the above
issue in better detail? Maybe Jon could also help here.

Also, I would recommend moving the two large _Safe_sequence functions to
a new safe_sequence.tcc (please stylistically double check the positions
of the open curly brackets in _M_transfer_from_if and elsewhere).

Thanks,
Paolo.