General improvements

[daxpedda]

Builds on #55.

• Implement all Rust traits for Ristretto255.
• Remove NonVerifiableServerEvaluateResult as it was holding only one type.
• Apply warn(missing_debug_implementations) and implement Debug on all public types.
• Remove leftover copy methods now that we are compiling without alloc.
• Implement all Rust traits for PreparedEvaluationElement and PreparedTscalar.
• Introduce more concrete types for complex returned Iterators in the public API.
• Check for zero scalars during Evaluate according to the spec.
• Move de/serialization of elements and scalars from GenericArrays to slices.
• Fix new_with_key would panic if bytes don't have the right length.
• Fix serde de/serialization to use the checked variants in Group.

[daxpedda]

I couldn't come up with a better name or description that actually explains the error to a user. I think InverseError doesn't make any sense to a user.

[kevinlewi]

Looks good overall, will merge!

[kevinlewi]

Oh oops, I probably should have re-run the CI before merging this, seems like the main branch now is failing CI. @daxpedda any ideas here?

[daxpedda]

Yes ... this is interesting and something I wasn't aware of, depending on pre-releases can automatically update. I thought that pre-releases can not update as they are considered unstable. derive-where is the offender here as it released a breaking change recently, which is fine because it's a pre-release, but downstream has to pin then.

This is addressed by #54, I'm gonna pin pre-release dependencies there.