• bench: switch to std::chrono for time measurements

std::chrono removes portability issues.

Rather than storing doubles, store the untouched time_points. Then
convert to nanoseconds for display. This allows for maximum precision, while
keeping results comparable between differing hardware/operating systems.

Also, display full nanosecond counts rather than sub-second floats.

• bench: prefer a steady clock if the resolution is no worse
• Require a steady clock for bench with at least micro precision

This is a backport of core PR11562 and PR11646