generalize-curve: move CurvePoint behind a trait

[naure]

In this PR we make the following changes:

• Introduce a trait for the curve (named CurveTrait).
• Introduce a trait for the scalar (temp. name ScalarTrait).
• Introduce a trait for the verifying key (named VerifyingKeyTrait).
• Introduce a trait for the Signature (temp. name SignatureTrait).
• Add the generic type (<C>) everywhere.
• Extract operations into the traits. This is not a full curve trait, rather it represents exactly the needs of the library.
• Add P256 curve implementation of the above 4 traits
• Generalize part of the testing infra structure
• Create variable TestCurve that points to the curve that will be fully tested
  (joint work with @emmorais)

[gatoWololo]

Hello,

I left some minor feedback. Overall this looks good to me. This change is very impressive. It adds quite a bit of complexity specially with the Generics + associated types and lifetimes. But it all seems to work. And IDK how we would do this any simpler.

My only real thought. I notice we have a pattern of C: CurveTrait + 'static throughout. I wonder if it would be possible to fold in the 'static lifetime as part of the definition on trait CurveTrait. This way we wouldn't need to list it in so many places (IDK this may not work). The observation is that we don't have a use case for our C: CurveTrait to ever have a non-static lifetime, i.e some reference.

[emmorais]

Hello,

I left some minor feedback. Overall this looks good to me. This change is very impressive. It adds quite a bit of complexity specially with the Generics + associated types and lifetimes. But it all seems to work. And IDK how we would do this any simpler.

My only real thought. I notice we have a pattern of C: CurveTrait + 'static throughout. I wonder if it would be possible to fold in the 'static lifetime as part of the definition on trait CurveTrait. This way we wouldn't need to list it in so many places (IDK this may not work). The observation is that we don't have a use case for our C: CurveTrait to ever have a non-static lifetime, i.e some reference.

Indeed moving static to CurveTrait allowed to remove a lot of repeated code. Thank you.

[jakinyele]

LGTM!