feat: make AffineRepr::xy() more flexible

[vmx]

Description

I decided to open a PR instead of an issue, to also illustrate that the impact on the current code base won't be that big (besides it being a breaking change).

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Currently AffineRepr::xy() returns the field elements as references. This is only possible if the underlying data structure that implements that traits owns those field elements and can directly have a reference to it.

Though there are cases, where providing those direct references is not possible. For example if the underlying structure has a different layout, or if the fields elements you're returning need to be wrapped in a newtype struct (that's my case).

With this change, AffineRepr::xy() now returns owned data. It's more flexible as now the underlying data structure can have any shape as long as it can return those field elements somehow.

closes: #XXXX

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[Pratyush]

Hm can you leave a snippet of your code here? Would like to see an example use case. Also, I believe the curve traits require the underlying data to be owned (ie any type implementing the AffineRepr trait must not contain any non-static references)

[vmx]

My actual code looks like this:

pub struct G1Affine(blstrs::G1Affine);

impl AffineRepr for G1Affine {
…
    fn xy(&self) -> Option<(Self::BaseField, Self::BaseField)> {
        if self.0.is_identity().into() {
            None
        } else {
            Some((Fp(self.0.x()), Fp(self.0.y())))
        }
    }
…
}

What it shows: I indeed own the underlying data. Though I'm not able to return a reference. One thing is that self.0.x() doesn't return a reference, but even if it would, I'd need to wrap it in a type (Fp in this case) that implements the arkworks traits. I can't do that directly on the blstrs types (without forking) as they would be foreign. This Fp cannot be a reference as it is only temporarily created.

[Pratyush]

This looks good, could you just add an entry in the CHANGELOG under the Breaking changes section? Thanks!

[vmx]

I've rebased it. If I should also squash the commits into a single one, let me know.

[Pratyush]

I can handle that, don't worry about it! Thanks :)