improve benchmark script

- simpler old runbench script
- best of 10 for more stable timings
- align and cleanup output
The command line doesn't allow me to specify -release as an option to dmd ("Unexpected 'l' when converting from type string to type uint").

Maybe it should be `runbench [-h|--help] [-v|--verbose] [-r n|--repeat=n] [<test_regex>] [<dflags>] [-- <runargs>]` and use getopt.config.stopOnFirstNonOption to seperate test options from dmd options.