Add array serialization impls

[ryanleh]

Description

This PR impls CanonicalSerialize and CanonicalDeserialize for arrays of up to size 32. This is useful for deriving these traits on structs which contain arrays.

I put Default and Copy bounds on CanonicalDeserialize in order to initialize the initial empty array. We could get rid of these bounds and use an Option<T>, but I believe this necessitates heap allocation in order to unwrap the Option<T> for the final result which I thought best to avoid. Let me know what y'all think is best.

I'll complete the other TODOs once I get some form of approval from maintainers to move forward (not sure who to ask for review 😁 )

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• Wrote unit tests
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[mmagician]

@ryanleh I recently started re-working an oldish PR to replace some parts of the code with const generics: I have a draft of the changes to bigint crate here: #372.
Perhaps it would make sense for this PR to also utilise const generics? I believe it would be analogous to this impl.

Otherwise Copy and Default trait bounds sound reasonable. I think most of the types from algebra that would make sense to put in an array have these implemented.

[ryanleh]

@mmagician Ah, had totally missed that const generics had stabilized!! 😁 That definitely makes more sense -- I updated the PR appropriately.

[Pratyush]

I believe we can use new array methods to eliminate the Default + Copy bounds:

Ok([(); N].map(|_| T::deserialize(&mut reader).unwrap()))

There is a downside that this panics immediately if there's an error, but that's not necessarily a bad thing. This might be the best we can do until this feature is stabilized.

[ryanleh]

Awesome! I changed de-serialization to do this, removed the extra bounds, and added a TODO to update things once the feature is stabilized.

[ryanleh]

For some reason the CanonicalSerialize impl is causing serialization tests for BTreeMap, BTreeSet, Tuple, Option, and Rc to panic..? Not sure what's going on lol I'll take a look in a bit.

[Pratyush]

For some reason the CanonicalSerialize impl is causing serialization tests for BTreeMap, BTreeSet, Tuple, Option, and Rc to panic..? Not sure what's going on lol I'll take a look in a bit.

Should be fixed in the latest commit