Add preprocessing capabilities

[kevaundray]

Problem

Each time we create a proof, we are re-generating the proving keys, which only need to be done when the program changes.

Solution

There are a few steps which need to be done:

• Add capability to save the proving and verifying keys, and create and verify proofs using them.
• Add a checksum into nargo which hashes the constraint system and checks whether it has changed.
• When you notice that the checksum has changed, regenerate the proving and verifying keys before creating a proof. If they have changed when you are verifying, we should tell the user that the program has changed and they need to regenerate the proofs and the proving and verifying keys
• Note: we may be able to allow the user to create proofs and verify proofs by ignoring the program and just looking at the ACIR+ Proving/Verifying keys. This seems orthogonal and so we can ignore it for now