Split RNG related things into its own file

This further removes 2 function declarations from the common path

include/internal/catch_test_case_registry_impl.cpp

@@ -10,6 +10,7 @@
 #include "catch_context.h"
 #include "catch_enforce.h"
 #include "catch_interfaces_registry_hub.h"
+#include "catch_random_number_generator.h"
 #include "catch_string_manip.h"
 #include "catch_test_case_info.h"
 
@@ -17,9 +18,6 @@
 
 namespace Catch {
 
-    RandomNumberGenerator::result_type RandomNumberGenerator::operator()( result_type n ) const { return std::rand() % n; }
-    RandomNumberGenerator::result_type RandomNumberGenerator::operator()() const { return std::rand() % max(); }
-
     std::vector<TestCase> sortTests( IConfig const& config, std::vector<TestCase> const& unsortedTestCases ) {
 
         std::vector<TestCase> sorted = unsortedTestCases;