base_convert: keep track of non-zero limbs and mul the minimum.

[github-actions]

On 2022-05-28 @recmo wrote in b0b5d54 “Mod names”:

keep track of non-zero limbs and mul the minimum.

        for digit in digits {
            if digit >= base {
                return Err(BaseConvertError::InvalidDigit(digit, base));
            }
            // Multiply by base.
            // OPT: keep track of non-zero limbs and mul the minimum.
            let mut carry: u128 = u128::from(digit);
            #[allow(clippy::cast_possible_truncation)]
            for limb in result.limbs.iter_mut() {
                carry += u128::from(*limb) * u128::from(base);
                *limb = carry as u64;

From src/base_convert.rs:93