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_commandline.cpp

@@ -196,7 +196,6 @@ namespace Catch {
             | Opt( setWaitForKeypress, "start|exit|both" )
                 ["--wait-for-keypress"]
                 ( "waits for a keypress before exiting" )
-#ifndef CATCH_CONFIG_DISABLE_BENCHMARKING 
             | Opt( config.benchmarkSamples, "samples" )
                 ["--benchmark-samples"]
                 ( "number of samples to collect (default: 100)" )
@@ -209,7 +208,6 @@ namespace Catch {
             | Opt( config.benchmarkNoAnalysis )
                 ["--benchmark-no-analysis"]
                 ( "perform only measurements; do not perform any analysis" )
-#endif // CATCH_CONFIG_DISABLE_BENCHMARKING
 			| Arg( config.testsOrTags, "test name|pattern|tags" )
                 ( "which test or tests to use" );