adding a starknet u256 type

[tdelabro]

Starknet offers 3 types that are not available in std rust: Felt252, u256, u512.

Felt252 has been taken care of by this crate.
I believe we should also provide something for the two others.
Both u256 and u512 are similar in their logic, so I will now talk uniquely of u256, but everything I say is also relevant for u512.

There is some design decision to be taken.

Do we want an actual type or a trait?

We could only offer a trait that allow to convert back an forth to the starknet representation of u256, which is a struct make of two u128 (low and high), and let the user decide which actual implementation he wants to use for the arithmetics operation on his type.

On the other hand we could offer an actual type, either by coding it ourselves or wrapping an existing lib (primitve-types, biguint, ...) while providing the required conversion methods.

What is your opinion on this? What have been your needs and solutions so far?

For my project I created a struct used for serialization/deserialization of starknet u256. And convert to primitive-types when I'm in need of doing actual computation.
What about you?

[tdelabro]

@AvivYossef-starkware what are your views on this at Starkware?
@gabrielbosio does the VM has any need for this?
@0xLucqs what about the exploration team?
@antiyro what about Madara?

[AvivYossef-starkware]

We have a lot of use in U256.
For example node index in the Patricia tree, we used existing libs like ethnum::U256

[tdelabro]

@AvivYossef-starkware are you happy with this lib?
Would you rather have a dedicated StarknetU256 struct with custom arithmetic implemented on it? Or would having a trait that allows for proper conversion and serde/de be enough for your need?
Especially if it allows you to keep use multiple different impl of u256 depending of your needs and constrains

[AvivYossef-starkware]

I don't think that we need a dedicated structure for it.
A trait that allows conversion and serde/de sounds enough.