* Choose test inputs more thoroughly and systematically.
* Check that `isqrt` and `checked_isqrt` have equivalent results for
signed types, either equivalent numerically or equivalent as a panic
and a `None`.
* Check that `isqrt` has numerically-equivalent results for unsigned
types and their `NonZero` counterparts.
* Reuse `ilog10` benchmarks, plus benchmarks that use a uniform
distribution.
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