Implement mock verification circuit for IVC

[str4d]

This will contain all of the necessary proof structures (e.g. deferreds), but none of the logic; everything will be unconstrained.

[str4d]

In a pairing with @ebfull, we discussed what this should look like. We want to build a gadget that verifies a proof, that takes in:

• An accumulator. (This can be added later when we finish modifying the plonk verifier API to support this)
• A TranscriptReader for extracting the proof (values).
• The public inputs for the proof being verified.
• A verification key, containing:
  • The constraint system corresponding to the proof being verified.
    • This is the post-optimisation circuit (i.e. including floor planning and selector optimisations). The structure for this needs to be well-defined, which means this blocks on Create separate VerifyingKey structures for pre- and post-optimisation circuits #643.
    • We need a way to extract all the details that affect the constraint system (e.g. gate information, constraints) from this, which may need to be part of the public API depending on how the traits and crates get arranged.
  • Any other circuit-specific data needed to verify.

The output of the gadget is:

• An accumulator. (This can be added later when we finish modifying the plonk verifier API to support this)
• A proof_is_valid bit.

Once this gadget exists, we should be able to complete #247 relatively quickly, to have an end-to-end mock IVC.

[therealyingtong]

We need a way to extract all the details that affect the constraint system (e.g. gate information, constraints) from this, which may need to be part of the public API depending on how the traits and crates get arranged.

Possible preview of API changes: privacy-scaling-explorations@3e4b27f