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_config.cpp

@@ -60,12 +60,10 @@ namespace Catch {
     bool Config::showInvisibles() const                { return m_data.showInvisibles; }
     Verbosity Config::verbosity() const                { return m_data.verbosity; }
 
-#ifndef CATCH_CONFIG_DISABLE_BENCHMARKING 
     bool Config::benchmarkNoAnalysis() const           { return m_data.benchmarkNoAnalysis; }
     int Config::benchmarkSamples() const               { return m_data.benchmarkSamples; }
     double Config::benchmarkConfidenceInterval() const { return m_data.benchmarkConfidenceInterval; }
     unsigned int Config::benchmarkResamples() const    { return m_data.benchmarkResamples; }
-#endif // CATCH_CONFIG_DISABLE_BENCHMARKING
 
     IStream const* Config::openStream() {
         return Catch::makeStream(m_data.outputFilename);