Release-0.2.4

Correcting some problems with mixed visibility settings in different projects.

This is causing issues with people who are using the -fvisibility flag in GCC, because GCC likes to instantiate vft's wherever the type is encountered, and we need to retain control of where such types are provided.

We've corrected as many of these as we possibly could, and then set -fvisibility=hidden for the few remaining warnings we were unable to address for now.