Should preprocess be two functions: generate_proving_key and generate_verification_key?

[phated]

Problem

Currently, we require a preprocess function on the ProofSystem trait that needs to return the proving key and verification key; however the verifier might only need a verification key. Alternatively, backends such as https://github.com/TomAFrench/acvm-backend-groth16 generate the pk and vk at the same time.

Proposed solution

Figure out if we want to split these or if it would cause too much friction when implementing various backends.

Alternatives considered

Leave it as is.

Additional context

See thread at #231 (comment) for more discussion

Submission Checklist

• Once I hit submit, I will assign this issue to the Project Board with the appropriate tags.

[kevaundray]

Generally they are generated at the same time in a one-off relative expensive operation. So I think its fine to have it as is.