Fix name clash problems with intrusive iterators.

When I recently tried to use the pretty printer for boost::intrusive::list in a new environment it just didn't work. Turned out the problem is a name clash. The name BoostIntrusiveListIterator was used for both the pretty printer and an internal helper. I don't know why things used to work anyway, but it doesn't matter. This commit fixes the problem (and I made the corresponding fix for boost::intrusive::set).

The pretty printer for boost::intrusive::set had a bug that caused a bogus element to be printed for an empty set. Also fixed.

I also took the opportunity to clean up some formatting issues local to boost::intrusive::set/boost::intrusive::list (mainly got rid of the camel case).