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_execution_plan.hpp

@@ -0,0 +1,58 @@
+/*
+ *  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)
+ */
+
+ // Execution plan
+
+#ifndef TWOBLUECUBES_CATCH_EXECUTION_PLAN_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_EXECUTION_PLAN_HPP_INCLUDED
+
+#include "../catch_config.hpp"
+#include "catch_clock.hpp"
+#include "catch_environment.hpp"
+#include "detail/catch_benchmark_function.hpp"
+#include "detail/catch_repeat.hpp"
+#include "detail/catch_run_for_at_least.hpp"
+
+#include <algorithm>
+
+namespace Catch {
+    namespace Benchmark {
+        template <typename Duration>
+        struct ExecutionPlan {
+            int iterations_per_sample;
+            Duration estimated_duration;
+            Detail::BenchmarkFunction benchmark;
+            Duration warmup_time;
+            int warmup_iterations;
+
+            template <typename Duration2>
+            operator ExecutionPlan<Duration2>() const {
+                return { iterations_per_sample, estimated_duration, benchmark, warmup_time, warmup_iterations };
+            }
+
+            template <typename Clock>
+            std::vector<FloatDuration<Clock>> run(const IConfig &cfg, Environment<FloatDuration<Clock>> env) const {
+                // warmup a bit
+                Detail::run_for_at_least<Clock>(std::chrono::duration_cast<ClockDuration<Clock>>(warmup_time), warmup_iterations, Detail::repeat(now<Clock>{}));
+
+                std::vector<FloatDuration<Clock>> times;
+                times.reserve(cfg.benchmarkSamples());
+                std::generate_n(std::back_inserter(times), cfg.benchmarkSamples(), [this, env] {
+                    Detail::ChronometerModel<Clock> model;
+                    this->benchmark(Chronometer(model, iterations_per_sample));
+                    auto sample_time = model.elapsed() - env.clock_cost.mean;
+                    if (sample_time < FloatDuration<Clock>::zero()) sample_time = FloatDuration<Clock>::zero();
+                    return sample_time / iterations_per_sample;
+                });
+                return times;
+            }
+        };
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_EXECUTION_PLAN_HPP_INCLUDED