Split RNG related things into its own file

This further removes 2 function declarations from the common path

include/internal/catch_run_context.cpp

@@ -1,6 +1,7 @@
 #include "catch_run_context.hpp"
 #include "catch_context.h"
 #include "catch_enforce.h"
+#include "catch_random_number_generator.h"
 #include "catch_stream.h"
 
 #include <cassert>