SafeInt moved to github

Has it really been 7 years since I last posted? Yikes - wonder if anyone will see this.

The main news is that CodePlex is kaput, and while SafeInt is archived, the archive unhelpfully stripped off the file names. So it has been moved to GitHub, which is much better. All of the history from CodePlex has been preserved, and while I was at it, I checked to be sure that it still compiles properly on gcc, clang and latest couple of Visual Studio versions.

Then I enhanced the test suite, made make files for gcc and clang, and prodded by Dan Jump, added constexpr support. This resulted in several changes, none that would matter at runtime. There was also a corner case bug in the routine that deals with initializing a SafeInt from floating point, and that is now fixed.

I'm even batting around the idea of submitting it to Boost.

Oh, and a link to GitHub would be a good idea, d'oh -