chrono: use default_clock, default_time_point, default_duration wherever possible
chrono: the difference between time_points must be explicitly converted to duration
chrono: tsc_clock has benn added which uses Time Stamp Counter (TSC)
tests: edited due to recent changes
private headers: some top-level header files have been renamed ("_impl" suffix)
private headers: json_dump header clean up
private headers: core header clean up
private headers: internal header clean up
private headers: accumulators headers clean up
private headers: headers with metrics measuring points have been renamed.
common measuring points header has been added
private headers: common handystats header has been removed