Split RNG related things into its own file

This further removes 2 function declarations from the common path

include/internal/catch_random_number_generator.cpp

@@ -0,0 +1,31 @@
+/*
+ *  Created by Martin on 30/08/2017.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_random_number_generator.h"
+#include "catch_context.h"
+#include "catch_interfaces_config.h"
+
+#include <cstdlib>
+
+namespace Catch {
+
+    void seedRng( IConfig const& config ) {
+        if( config.rngSeed() != 0 )
+            std::srand( config.rngSeed() );
+    }
+    unsigned int rngSeed() {
+        return getCurrentContext().getConfig()->rngSeed();
+    }
+
+    RandomNumberGenerator::result_type RandomNumberGenerator::operator()( result_type n ) const {
+        return std::rand() % n;
+    }
+    RandomNumberGenerator::result_type RandomNumberGenerator::operator()() const {
+        return std::rand() % max();
+    }
+
+}