pub(crate) fn is_8digits(v: u64) -> bool {
    let a = v.wrapping_add(0x4646_4646_4646_4646);
    let b = v.wrapping_sub(0x3030_3030_3030_3030);
    (a | b) & 0x8080_8080_8080_8080 == 0
}
Determine if 8 bytes are all decimal digits.
This does not care about the order in which the bytes were loaded.
sonic-number/src/common.rs