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/detail/catch_estimate_clock.hpp

@@ -0,0 +1,113 @@
+/*
+ *  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)
+ */
+
+ // Environment measurement
+
+#ifndef TWOBLUECUBES_CATCH_DETAIL_ESTIMATE_CLOCK_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_DETAIL_ESTIMATE_CLOCK_HPP_INCLUDED
+
+#include "../catch_clock.hpp"
+#include "../catch_environment.hpp"
+#include "catch_stats.hpp"
+#include "catch_measure.hpp"
+#include "catch_run_for_at_least.hpp"
+#include "../catch_clock.hpp"
+
+#include <algorithm>
+#include <iterator>
+#include <tuple>
+#include <vector>
+#include <cmath>
+
+namespace Catch {
+    namespace Benchmark {
+        namespace Detail {
+            template <typename Clock>
+            std::vector<double> resolution(int k) {
+                std::vector<TimePoint<Clock>> times;
+                times.reserve(k + 1);
+                std::generate_n(std::back_inserter(times), k + 1, now<Clock>{});
+
+                std::vector<double> deltas;
+                deltas.reserve(k);
+                std::transform(std::next(times.begin()), times.end(), times.begin(),
+                    std::back_inserter(deltas),
+                    [](TimePoint<Clock> a, TimePoint<Clock> b) { return static_cast<double>((a - b).count()); });
+
+                return deltas;
+            }
+
+            const auto warmup_iterations = 10000;
+            const auto warmup_time = std::chrono::milliseconds(100);
+            const auto minimum_ticks = 1000;
+            const auto warmup_seed = 10000;
+            const auto clock_resolution_estimation_time = std::chrono::milliseconds(500);
+            const auto clock_cost_estimation_time_limit = std::chrono::seconds(1);
+            const auto clock_cost_estimation_tick_limit = 100000;
+            const auto clock_cost_estimation_time = std::chrono::milliseconds(10);
+            const auto clock_cost_estimation_iterations = 10000;
+
+            template <typename Clock>
+            int warmup() {
+                return run_for_at_least<Clock>(std::chrono::duration_cast<ClockDuration<Clock>>(warmup_time), warmup_seed, &resolution<Clock>)
+                    .iterations;
+            }
+            template <typename Clock>
+            EnvironmentEstimate<FloatDuration<Clock>> estimate_clock_resolution(int iterations) {
+                auto r = run_for_at_least<Clock>(std::chrono::duration_cast<ClockDuration<Clock>>(clock_resolution_estimation_time), iterations, &resolution<Clock>)
+                    .result;
+                return {
+                    FloatDuration<Clock>(mean(r.begin(), r.end())),
+                    classify_outliers(r.begin(), r.end()),
+                };
+            }
+            template <typename Clock>
+            EnvironmentEstimate<FloatDuration<Clock>> estimate_clock_cost(FloatDuration<Clock> resolution) {
+                auto time_limit = std::min(resolution * clock_cost_estimation_tick_limit, FloatDuration<Clock>(clock_cost_estimation_time_limit));
+                auto time_clock = [](int k) {
+                    return Detail::measure<Clock>([k] {
+                        for (int i = 0; i < k; ++i) {
+                            volatile auto ignored = Clock::now();
+                            (void)ignored;
+                        }
+                    }).elapsed;
+                };
+                time_clock(1);
+                int iters = clock_cost_estimation_iterations;
+                auto&& r = run_for_at_least<Clock>(std::chrono::duration_cast<ClockDuration<Clock>>(clock_cost_estimation_time), iters, time_clock);
+                std::vector<double> times;
+                int nsamples = static_cast<int>(std::ceil(time_limit / r.elapsed));
+                times.reserve(nsamples);
+                std::generate_n(std::back_inserter(times), nsamples, [time_clock, &r] {
+                    return static_cast<double>((time_clock(r.iterations) / r.iterations).count());
+                });
+                return {
+                    FloatDuration<Clock>(mean(times.begin(), times.end())),
+                    classify_outliers(times.begin(), times.end()),
+                };
+            }
+
+            template <typename Clock>
+            Environment<FloatDuration<Clock>> measure_environment() {
+                static Environment<FloatDuration<Clock>>* env = nullptr;
+                if (env) {
+                    return *env;
+                }
+
+                auto iters = Detail::warmup<Clock>();
+                auto resolution = Detail::estimate_clock_resolution<Clock>(iters);
+                auto cost = Detail::estimate_clock_cost<Clock>(resolution.mean);
+
+                env = new Environment<FloatDuration<Clock>>{ resolution, cost };
+                return *env;
+            }
+        } // namespace Detail
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_DETAIL_ESTIMATE_CLOCK_HPP_INCLUDED