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_measure.hpp

@@ -0,0 +1,35 @@
+/*
+ *  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)
+ */
+
+// Measure
+
+#ifndef TWOBLUECUBES_CATCH_DETAIL_MEASURE_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_DETAIL_MEASURE_HPP_INCLUDED
+
+#include "../catch_clock.hpp"
+#include "catch_complete_invoke.hpp"
+#include "catch_timing.hpp"
+
+#include <utility>
+
+namespace Catch {
+    namespace Benchmark {
+        namespace Detail {
+            template <typename Clock, typename Fun, typename... Args>
+            TimingOf<Clock, Fun(Args...)> measure(Fun&& fun, Args&&... args) {
+                auto start = Clock::now();
+                auto&& r = Detail::complete_invoke(fun, std::forward<Args>(args)...);
+                auto end = Clock::now();
+                auto delta = end - start;
+                return { delta, std::forward<decltype(r)>(r), 1 };
+            }
+        } // namespace Detail
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_DETAIL_MEASURE_HPP_INCLUDED