FYI: update libstdc++ pretty-printers

[Benjamin Kosnik]

> Doug Evans recently added some features to gdb to make it possible for
> the user to individually enable or disable pretty-printers.  In order
> for this to work, the printers must conform to a certain extended
> protocol.

By this do you mean type-specific enable/disable? Ie, enable
std::string prettyprinter, and have std::list disabled? 

If so, way cool, can you give example syntax that does it?

> This patch updates the libstdc++ printers to work this way.  The
> change is done in a backward-compatible way, so that they will
> continue to work properly in older versions of gdb.
> 
> This patch also simplifies the printers somewhat, and probably speeds
> up printer recognition as well, though I did not try to measure that.

Nice job on the simplification. I find the new version easier to read,
FWIW. Registration seems much simpler.

-benjamin