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/catch_meta.hpp

@@ -12,8 +12,23 @@
 #include <type_traits>
 
 namespace Catch {
-    template<typename T>
-    struct always_false : std::false_type {};
+template<typename T>
+struct always_false : std::false_type {};
+
+template <typename> struct true_given : std::true_type {};
+struct is_callable_tester {
+    template <typename Fun, typename... Args>
+    true_given<decltype(std::declval<Fun>()(std::declval<Args>()...))> static test(int);
+    template <typename...>
+    std::false_type static test(...);
+};
+
+template <typename T>
+struct is_callable;
+
+template <typename Fun, typename... Args>
+struct is_callable<Fun(Args...)> : decltype(is_callable_tester::test<Fun, Args...>(0)) {};
+
 } // namespace Catch
 
 #endif // TWOBLUECUBES_CATCH_META_HPP_INCLUDED