It uses random to determine if the bignum is sparse or not.
It is arguable if three-digit samples are enough or not to determine it,
but anyway, consuming Random source implicitly is not good.

I introduced the random sampling mechanism, and I don't know any
significant reason to do so.  So, let's remove it.
This change makes the sampling points fixed: 40th, 50th, and 60th
percentiles.