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_random_number_generator.h

@@ -7,17 +7,52 @@
 #ifndef TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED
 #define TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED
 
-#include <algorithm>
-#include <random>
+#include <cstdint>
 
 namespace Catch {
 
-    struct IConfig;
+    // This is a simple implementation of C++11 Uniform Random Number
+    // Generator. It does not provide all operators, because Catch2
+    // does not use it, but it should behave as expected inside stdlib's
+    // distributions.
+    // The implementation is based on the PCG family (http://pcg-random.org)
+    class SimplePcg32 {
+        using state_type = std::uint64_t;
+    public:
+        using result_type = std::uint32_t;
+        static constexpr result_type min() {
+            return 0;
+        }
+        static constexpr result_type max() {
+            return static_cast<result_type>(-1);
+        }
 
-    std::mt19937& rng();
-    void seedRng( IConfig const& config );
-    unsigned int rngSeed();
+        // Provide some default initial state for the default constructor
+        SimplePcg32():SimplePcg32(0xed743cc4U) {}
 
-}
+        explicit SimplePcg32(result_type seed_);
+
+        void seed(result_type seed_);
+        void discard(uint64_t skip);
+
+        result_type operator()();
+
+    private:
+        friend bool operator==(SimplePcg32 const& lhs, SimplePcg32 const& rhs);
+        friend bool operator!=(SimplePcg32 const& lhs, SimplePcg32 const& rhs);
+
+        // In theory we also need operator<< and operator>>
+        // In practice we do not use them, so we will skip them for now
+
+
+        std::uint64_t m_state;
+        // This part of the state determines which "stream" of the numbers
+        // is chosen -- we take it as a constant for Catch2, so we only
+        // need to deal with seeding the main state.
+        // Picked by reading 8 bytes from `/dev/random` :-)
+        static const std::uint64_t s_inc = (0x13ed0cc53f939476ULL << 1ULL) | 1ULL;
+    };
+
+} // end namespace Catch
 
 #endif // TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED