Fix some _GLIBCXX_DEBUG pretty printer errors

[François Dumont]

Hi

 Â Â Â  Here is a patch to fix some pretty printer check errors when 
running those tests with _GLIBCXX_DEBUG.

 Â Â Â  I introduced a special rendered for the std::forward_list iterator 
which is similar to the one used for the std::list and so used 
inheritance, I hope it is not a problem for Python 2/3 compatibility.

 Â Â Â  I prefer to just rely on normal iterator rendered when we detect 
that iterator hasn't been initialized so that normal or debug modes give 
the same result.

 Â Â Â  Ok to commit ?

François


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