Various improvements to the benchmarking support

* Units from <ratio> are no longer redeclared in our own namespace
* The default clock is `steady_clock`, not `high_resolution_clock`,
because, as HH says "high_resolution_clock is useless. If you want
measure the passing of time, use steady_clock. If you want user
friendly time, use system_clock".
* Benchmarking support is opt-in, not opt-out, to avoid the large
(~10%) compile time penalty.
* Benchmarking-related options in CLI are always present, to decrease
the amount of code that is only compiled conditionally and making
the whole shebang more maintainble.

projects/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp

@@ -7,7 +7,7 @@
  */
 
 #include "catch.hpp"
-#ifndef CATCH_CONFIG_DISABLE_BENCHMARKING 
+#if defined(CATCH_CONFIG_ENABLE_BENCHMARKING)
 namespace {
     struct manual_clock {
     public:
@@ -402,4 +402,4 @@ TEST_CASE("run benchmark", "[benchmark]") {
 
     CHECK((end - start).count() == 2867251000);
 }
-#endif // CATCH_CONFIG_DISABLE_BENCHMARKING
+#endif // CATCH_CONFIG_ENABLE_BENCHMARKING