log: Find out exactly where the precision is lost and what

[github-actions]

On 2022-05-31 @recmo wrote in c1a0c34 “Merge pull request #97 from recmo/isqrt”:

Find out exactly where the precision is lost and what
the bounds are.

                let log = value.approx_log2();
                let pow = U::approx_pow2(log).unwrap();
                let error = value.abs_diff(pow);
                let correct_bits = value.bit_len() - error.bit_len();
                // The maximum precision we could expect here is 53 bits.
                // OPT: Find out exactly where the precision is lost and what
                // the bounds are.
                assert!(correct_bits == value.bit_len() || correct_bits >= 42);
            });
        });
    }

From src/log.rs:150