Integrate Nonius benchmark into Catch2

Changes done to Nonius:
* Moved things into "Catch::Benchmark" namespace
* Benchmarks were integrated with `TEST_CASE`/`SECTION`/`GENERATE` macros
* Removed Nonius's parameters for benchmarks, Generators should be used instead
* Added relevant methods to the reporter interface (default-implemented, to avoid
breaking existing 3rd party reporters)
* Async processing is guarded with `_REENTRANT` macro for GCC/Clang, used by default
on MSVC
* Added a macro `CATCH_CONFIG_DISABLE_BENCHMARKING` that removes all traces of
benchmarking from Catch

include/internal/benchmark/catch_clock.hpp

@@ -0,0 +1,46 @@
+/*
+ *  Created by Joachim on 16/04/2019.
+ *  Adapted from donated nonius code.
+ *
+ *  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)
+ */
+
+// Clocks
+
+#ifndef TWOBLUECUBES_CATCH_CLOCK_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_CLOCK_HPP_INCLUDED
+
+#include <chrono>
+#include <ratio>
+
+namespace Catch {
+    namespace Benchmark {
+        template <unsigned Num, unsigned Den = 1>
+        using ratio = std::ratio<Num, Den>;
+        using milli = ratio<1, 1000>;
+        using micro = ratio<1, 1000000>;
+        using nano = ratio<1, 1000000000>;
+
+        template <typename Clock>
+        using ClockDuration = typename Clock::duration;
+        template <typename Clock>
+        using FloatDuration = std::chrono::duration<double, typename Clock::period>;
+
+        template <typename Clock>
+        using TimePoint = typename Clock::time_point;
+
+        using default_clock = std::chrono::high_resolution_clock;
+
+        template <typename Clock>
+        struct now {
+            TimePoint<Clock> operator()() const {
+                return Clock::now();
+            }
+        };
+
+        using fp_seconds = std::chrono::duration<double, ratio<1>>;
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_CLOCK_HPP_INCLUDED