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_optimizer.hpp

@@ -0,0 +1,68 @@
+/*
+ *  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)
+ */
+
+ // Hinting the optimizer
+
+#ifndef TWOBLUECUBES_CATCH_OPTIMIZER_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_OPTIMIZER_HPP_INCLUDED
+
+#if defined(_MSC_VER)
+#   include <atomic> // atomic_thread_fence
+#endif
+
+namespace Catch {
+    namespace Benchmark {
+#if defined(__GNUC__) || defined(__clang__)
+        template <typename T>
+        inline void keep_memory(T* p) {
+            asm volatile("" : : "g"(p) : "memory");
+        }
+        inline void keep_memory() {
+            asm volatile("" : : : "memory");
+        }
+
+        namespace Detail {
+            inline void optimizer_barrier() { keep_memory(); }
+        } // namespace Detail
+#elif defined(_MSC_VER)
+
+#pragma optimize("", off)
+        template <typename T>
+        inline void keep_memory(T* p) {
+            // thanks @milleniumbug
+            *reinterpret_cast<char volatile*>(p) = *reinterpret_cast<char const volatile*>(p);
+        }
+        // TODO equivalent keep_memory()
+#pragma optimize("", on)
+
+        namespace Detail {
+            inline void optimizer_barrier() {
+                std::atomic_thread_fence(std::memory_order_seq_cst);
+            }
+        } // namespace Detail
+
+#endif
+
+        template <typename T>
+        inline void deoptimize_value(T&& x) {
+            keep_memory(&x);
+        }
+
+        template <typename Fn, typename... Args>
+        inline auto invoke_deoptimized(Fn&& fn, Args&&... args) -> typename std::enable_if<!std::is_same<void, decltype(fn(args...))>::value>::type {
+            deoptimize_value(std::forward<Fn>(fn) (std::forward<Args...>(args...)));
+        }
+
+        template <typename Fn, typename... Args>
+        inline auto invoke_deoptimized(Fn&& fn, Args&&... args) -> typename std::enable_if<std::is_same<void, decltype(fn(args...))>::value>::type {
+            std::forward<Fn>(fn) (std::forward<Args...>(args...));
+        }
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_OPTIMIZER_HPP_INCLUDED