A better implementation for `RationalU256`.

## Issue

Panicked at 'U256: attempt to multiply with overflow' for some values.

https://github.com/nervosnetwork/ckb/blob/649389d90a142cf39cdbc2019b8fbde188383ca5/util/rational/src/lib.rs#L128-L140

### Reproduce

Multiply two `RationalU256` numbers, when both two numerators (or denominators) are big but their greatest common divisor is small.

For example:

```rust
use ckb_types::{core::RationalU256, u256, U256};
fn main() {
let x = u256!("0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
let x_rational = RationalU256::from_u256(x.clone());
let numerator = 33u32;
let denominator = 100u32;
let y_rational = RationalU256::new(U256::from(numerator), U256::from(denominator));
let z_rational = x_rational * y_rational; // panicked in this line!
let z = z_rational.into_u256();
println!("{:#x} = {:#x} * {}/{}", z, x, numerator, denominator);
}
```

Ref: nervosnetwork/ckb-light-client#44

Start in util/rational/src/lib.rs at lines 128-140 and reproduce the panic with the example using RationalU256 and large U256 values. Read the multiplication path and inspect how the greatest common divisor is handled when numerators or denominators are large. Done means the example no longer panics and produces the expected value from into_u256().

Written by the indexing model from the issue text.