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/CmdLine.tests.cpp

@@ -463,7 +463,6 @@ TEST_CASE( "Process can be configured on command line", "[config][command-line]"
         }
     }
 
-#ifndef CATCH_CONFIG_DISABLE_BENCHMARKING 
     SECTION("Benchmark options") {
         SECTION("samples") {
             CHECK(cli.parse({ "test", "--benchmark-samples=200" }));
@@ -489,5 +488,4 @@ TEST_CASE( "Process can be configured on command line", "[config][command-line]"
             REQUIRE(config.benchmarkNoAnalysis);
         }
     }
-#endif
 }