Zcash-style serialization for BLS12-381

[mmagician]

Description

Adapted from https://github.com/crate-crypto/small-powers-of-tau/blob/master/src/interop_point_encoding.rs to take advantage of flexible serialization for curves.

Question: should we maybe have this mode of serialization feature-gated, e.g.:

...
#[cfg(feature = "zcash_serialization")]
fn serialized_size(_compress: ark_serialize::Compress) -> usize {
    G2_SERIALISED_SIZE
}
...

---

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• Targeted PR against correct branch (master)
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• Wrote unit tests
• Updated relevant documentation in the code
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[mmagician]

@Pratyush Thanks for the review, these are of course valid points 😉
As you've seen, I've opened another fix for get_point_from_x_unchecked - this PR depends on it for the tests to pass.

[Pratyush]

btw I think there's no need to feature-gate the new compression. We can just leave a note in the CHANGELOG about this being a breaking change.

[daira]

Note that Zcash always uses compressed point encodings, so the uncompressed version should not be referred to as Zcash-style encoding in documentation.

[mmagician]

@daira Noted, thanks. So far it's not mentioned in the docs, but will go into the CHANGELOG. I've just reworded it there, I hope it's fine like that?