[feat] Implement Hash trait for OutPoint

[m-kus]

We will need to calculate hashes of outpoints to update Utreexo accumulator and local cache.

Hash trait: https://github.com/starkware-libs/cairo/blob/93221753088d58f54f9a7f35a0bb338cf0bfb952/corelib/src/hash.cairo#L13

Hints:

• Rename hash::Hash to Digest to avoid confusion
• Use Serde::serialize to convert Outpoint into array of felts

Suggested deadline: 9 Sep

[ScottyDavies]

@m-kus can you please assign this issue to me?

[m-kus]

How is the progress @ScottyDavies ? Do you need help with this task?

[ScottyDavies]

It’s going fine, I would be done soon

[maciejka]

@ScottyDavies how is it going? Do you need any help? Please let us know when do you expect to complete this issue.

[dlaciport]

Can I work on this issue? My PR #142 is almost done.

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[m-kus]

@bloomingpeach ideally you'd need to wait for #144 to be merged to use it for serialization

[dlaciport]

Hi, @m-kus, could you help me a little bit on this issue? what hash function should I use and should I introduce a new struct for the purpose of implement the HashState trait?

[m-kus]

@bloomingpeach Hash trait is agnostic to the particular hash function, it just allows to update hash state with a value of a given type. You can use Poseidon in unit tests for example.

You can find more info here: https://book.cairo-lang.org/ch11-04-hash.html#working-with-hashes

Here is an example of how Hash is implemented for a new type: https://github.com/starkware-libs/cairo/blob/004d2b4627b6a06a72f837957f0970cdd49d01d7/corelib/src/hash.cairo#L92

Note that you can also derive hash like #[derive(Hash)] as long as all members also implement it.
Maybe it will be enough to just implement Hash for @ByteArray (see how Serde is implemented for @ByteArray in types::transaction) and derive for the rest of the types.