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.

include/internal/catch_interfaces_config.h

@@ -77,12 +77,10 @@ namespace Catch {
         virtual std::vector<std::string> const& getSectionsToRun() const = 0;
         virtual Verbosity verbosity() const = 0;
 
-#ifndef CATCH_CONFIG_DISABLE_BENCHMARKING 
         virtual bool benchmarkNoAnalysis() const = 0;
         virtual int benchmarkSamples() const = 0;
         virtual double benchmarkConfidenceInterval() const = 0;
         virtual unsigned int benchmarkResamples() const = 0;
-#endif // CATCH_CONFIG_DISABLE_BENCHMARKING
     };
 
     using IConfigPtr = std::shared_ptr<IConfig const>;