feat: u256 support

[tdelabro]

Feature Request

Describe the Feature Request

Support for u256

Describe Preferred Solution

I would like for this lib to expose a U256 type, made of two felts, low and high.

pub struct U256 {
  low: Felt,
  high: Felt,
}

• lot of quality of like conversion methods: From, FromStr, From<(u128, u128)>, TryInto, TryInto<[Felt, 2]> and so on

Alternative Solution

Some more efficient internal representation, but keeping the ability transform from/to two felts when the need arise

Related Code

I have to use https://docs.rs/starknet/0.13.0/starknet/core/types/struct.U256.html instead. Which doesn't do a great job. So I instead have to handle tupples or custom structs to do this