Error order for comparing Decimal128 or Decimal256

[liukun4515]

Describe the bug
Decimal128 as a example, we use the [u8;16] to store the value of the decimal.

In order to compare the decimal128, we should use the signed compare order.

But the current implementation just compare the [u8;16] to get the order.

For example:
Decimal128(-1, 10,0).cmp(Decimal128(129,10,0) will get the result of Greater.

To Reproduce

    #[test]
    fn compare_i128() {
        let v: i128 = -1;
        let b1: [u8; 16] = v.to_le_bytes();
        let b2: [u8; 16] = 129_i128.to_le_bytes();
        println!("{:?}", b1);
        println!("{:?}", b2);
        if b1.eq(&b2) {
            println!("=")
        }
        println!("{:?}", b1.cmp(&b2));
    }

Expected behavior

Additional context

[liukun4515]

FYI @viirya
I will fix it.

[viirya]

Good catch!