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_benchmark_function.hpp

@@ -0,0 +1,105 @@
+    /*
+ *  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)
+ */
+
+ // Dumb std::function implementation for consistent call overhead
+
+#ifndef TWOBLUECUBES_CATCH_DETAIL_BENCHMARK_FUNCTION_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_DETAIL_BENCHMARK_FUNCTION_HPP_INCLUDED
+
+#include "../catch_chronometer.hpp"
+#include "catch_complete_invoke.hpp"
+#include "../../catch_meta.hpp"
+
+#include <cassert>
+#include <type_traits>
+#include <utility>
+#include <memory>
+
+namespace Catch {
+    namespace Benchmark {
+        namespace Detail {
+            template <typename T>
+            using Decay = typename std::decay<T>::type;
+            template <typename T, typename U>
+            struct is_related
+                : std::is_same<Decay<T>, Decay<U>> {};
+
+            /// We need to reinvent std::function because every piece of code that might add overhead
+            /// in a measurement context needs to have consistent performance characteristics so that we
+            /// can account for it in the measurement.
+            /// Implementations of std::function with optimizations that aren't always applicable, like
+            /// small buffer optimizations, are not uncommon.
+            /// This is effectively an implementation of std::function without any such optimizations;
+            /// it may be slow, but it is consistently slow.
+            struct BenchmarkFunction {
+            private:
+                struct callable {
+                    virtual void call(Chronometer meter) const = 0;
+                    virtual callable* clone() const = 0;
+                    virtual ~callable() = default;
+                };
+                template <typename Fun>
+                struct model : public callable {
+                    model(Fun&& fun) : fun(std::move(fun)) {}
+                    model(Fun const& fun) : fun(fun) {}
+
+                    model<Fun>* clone() const override { return new model<Fun>(*this); }
+
+                    void call(Chronometer meter) const override {
+                        call(meter, is_callable<Fun(Chronometer)>());
+                    }
+                    void call(Chronometer meter, std::true_type) const {
+                        fun(meter);
+                    }
+                    void call(Chronometer meter, std::false_type) const {
+                        meter.measure(fun);
+                    }
+
+                    Fun fun;
+                };
+
+                struct do_nothing { void operator()() const {} };
+
+                template <typename T>
+                BenchmarkFunction(model<T>* c) : f(c) {}
+
+            public:
+                BenchmarkFunction()
+                    : f(new model<do_nothing>{ {} }) {}
+
+                template <typename Fun,
+                    typename std::enable_if<!is_related<Fun, BenchmarkFunction>::value, int>::type = 0>
+                    BenchmarkFunction(Fun&& fun)
+                    : f(new model<typename std::decay<Fun>::type>(std::forward<Fun>(fun))) {}
+
+                BenchmarkFunction(BenchmarkFunction&& that)
+                    : f(std::move(that.f)) {}
+
+                BenchmarkFunction(BenchmarkFunction const& that)
+                    : f(that.f->clone()) {}
+
+                BenchmarkFunction& operator=(BenchmarkFunction&& that) {
+                    f = std::move(that.f);
+                    return *this;
+                }
+
+                BenchmarkFunction& operator=(BenchmarkFunction const& that) {
+                    f.reset(that.f->clone());
+                    return *this;
+                }
+
+                void operator()(Chronometer meter) const { f->call(meter); }
+
+            private:
+                std::unique_ptr<callable> f;
+            };
+        } // namespace Detail
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_DETAIL_BENCHMARK_FUNCTION_HPP_INCLUDED