Split RNG related things into its own file

This further removes 2 function declarations from the common path

include/internal/catch_random_number_generator.h

@@ -0,0 +1,40 @@
+/*
+ *  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)
+ */
+#ifndef TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED
+#define TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED
+
+#include "catch_random_number_generator.h"
+
+#include <algorithm>
+
+namespace Catch {
+
+    struct IConfig;
+
+    void seedRng( IConfig const& config );
+
+    unsigned int rngSeed();
+
+    struct RandomNumberGenerator {
+        using result_type = std::ptrdiff_t;
+
+        static constexpr result_type min() { return 0; }
+        static constexpr result_type max() { return 1000000; }
+
+        result_type operator()( result_type n ) const;
+        result_type operator()() const;
+
+        template<typename V>
+        static void shuffle( V& vector ) {
+            RandomNumberGenerator rng;
+            std::shuffle( vector.begin(), vector.end(), rng );
+        }
+    };
+
+}
+
+#endif // TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED