Kill `CurveExt` and `CurveAffine` traits

[str4d]

These were helper traits that we created to provide additional methods needed for halo2_proofs that the group crate traits didn't have, while we figured out what those needed additional methods were. Now that we've cut halo2_proofs 0.1.0, we need to finish the job:

• Figure out how to better abstract these additional methods.
• Add these abstractions to group.
• Delete the CurveExt and CurveAffine traits.