Introduce Catch's own RNG based on the PCG family of RNGs

In the future, we will also want to introduce our own
`uniform_int_distribution` and `uniform_real_distribution` to get
repeatable test runs across different platforms.

include/internal/catch_run_context.h

@@ -151,6 +151,8 @@ namespace Catch {
         bool m_includeSuccessfulResults;
     };
 
+    void seedRng(IConfig const& config);
+    unsigned int rngSeed();
 } // end namespace Catch
 
 #endif // TWOBLUECUBES_CATCH_RUNNER_IMPL_HPP_INCLUDED