[End-user workaround] Constantine-rust: public fields and Default fields

[mratsim]

Requested by https://github.com/sifraitech/rust-kzg

Due to the orphan rule preventing building traits on top of external libraries, the Sifraitech team needs the fields to be public.

Unfortunately this conflicts with security best practices and isolation concerns, especially given that Constantine may handle sensitive secrets that must be protected in a best effort manner. Keeping fields private is a good way to avoid accidentally spilling data in a debug log for example.

This exposes the fields in a branch while keeping the library pristine.

This also adds a Default trait implementation for convenience. For elliptic curves using it as-is MAY result in undefined behavior for Jacobian or Projective points, as setInf is the proper initialization.
This commit Ensure the projective point at infinity is not all zeros, but (0, 1, 0) fixed an issue with all zero initialization.
Though I'm not sure how to automate it with bindgen.

cc @Armantidas @sauliusgrigaitis

[mratsim]

If you use the KZG parallel implementation of Constantine, I have changed the API in #394.

The threadpool is now directly stored in the KZG context.

See https://github.com/mratsim/constantine/pull/394/files#diff-2c8630f256e9d494d19bbb20fae5cd7a6ced97a9800d5a192c1979bde72e5494R487

I can update the branch whenever.

No impact on the serial KZG.

[sauliusgrigaitis]

Thanks, if you want to work on rust-kzg update then check https://github.com/ArtiomTr/rust-kzg/tree/Integrate_sppark_msm . This is going to be merged soon. It also contains the updated performance graphs. Seems constantine peformance penalty is gone!