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_chronometer.hpp

@@ -0,0 +1,71 @@
+/*
+ *  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)
+ */
+
+// User-facing chronometer
+
+#ifndef TWOBLUECUBES_CATCH_CHRONOMETER_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_CHRONOMETER_HPP_INCLUDED
+
+#include "catch_clock.hpp"
+#include "catch_optimizer.hpp"
+#include "detail/catch_complete_invoke.hpp"
+#include "../catch_meta.hpp"
+
+namespace Catch {
+    namespace Benchmark {
+        namespace Detail {
+            struct ChronometerConcept {
+                virtual void start() = 0;
+                virtual void finish() = 0;
+                virtual ~ChronometerConcept() = default;
+            };
+            template <typename Clock>
+            struct ChronometerModel final : public ChronometerConcept {
+                void start() override { started = Clock::now(); }
+                void finish() override { finished = Clock::now(); }
+
+                ClockDuration<Clock> elapsed() const { return finished - started; }
+
+                TimePoint<Clock> started;
+                TimePoint<Clock> finished;
+            };
+        } // namespace Detail
+
+        struct Chronometer {
+        public:
+            template <typename Fun>
+            void measure(Fun&& fun) { measure(std::forward<Fun>(fun), is_callable<Fun(int)>()); }
+
+            int runs() const { return k; }
+
+            Chronometer(Detail::ChronometerConcept& meter, int k)
+                : impl(&meter)
+                , k(k) {}
+
+        private:
+            template <typename Fun>
+            void measure(Fun&& fun, std::false_type) {
+                measure([&fun](int) { return fun(); }, std::true_type());
+            }
+
+            template <typename Fun>
+            void measure(Fun&& fun, std::true_type) {
+                Detail::optimizer_barrier();
+                impl->start();
+                for (int i = 0; i < k; ++i) invoke_deoptimized(fun, i);
+                impl->finish();
+                Detail::optimizer_barrier();
+            }
+
+            Detail::ChronometerConcept* impl;
+            int k;
+        };
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_CHRONOMETER_HPP_INCLUDED