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_environment.hpp

@@ -0,0 +1,38 @@
+/*
+ *  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 information
+
+#ifndef TWOBLUECUBES_CATCH_ENVIRONMENT_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_ENVIRONMENT_HPP_INCLUDED
+
+#include "catch_clock.hpp"
+#include "catch_outlier_classification.hpp"
+
+namespace Catch {
+    namespace Benchmark {
+        template <typename Duration>
+        struct EnvironmentEstimate {
+            Duration mean;
+            OutlierClassification outliers;
+
+            template <typename Duration2>
+            operator EnvironmentEstimate<Duration2>() const {
+                return { mean, outliers };
+            }
+        };
+        template <typename Clock>
+        struct Environment {
+            using clock_type = Clock;
+            EnvironmentEstimate<FloatDuration<Clock>> clock_resolution;
+            EnvironmentEstimate<FloatDuration<Clock>> clock_cost;
+        };
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_ENVIRONMENT_HPP_INCLUDED