Add isc_histo for histogram statistics

Add isc_histo for histogram statistics

This is an adaptation of my `hg64` experiments for use in BIND.

As well as renaming everything according to ISC style, I have
written some more extensive tests that ensure the edge cases are
correct and the fenceposts are in the right places.

I have added utility functions for working with precision in terms of
decimal significant figures as well as this code's native binary.