Compact groth16 arguments

[dtebbs]

Compact the proof and vk into single arrays of evm words. Saves gas and reduces the number of parameters required in applictions where vk and proof must be passed as arguments.

Depends on #287

[AntoineRondelet]

Side note: Now that we support several curves (i.e. bn/bls) may be worth to clarify these comments. This contract is tailored for bn254, and won't work with any other groups. So May be worth specifying under with circumstances your comment above holds (i.e. ALT_BN128 set in cpp code config) or simply rephrase to make this generic, e.g.:

// Constructor. The form of vk is:
//     alpha = vk[:2*N]
//     beta = vk[2*N:6*N]
//     delta = vk[6*N:10*N]
//     ABC_coords = vk[10*N:]
//  where:
//     - q is the characteristic of the base field of the pairing group used
//     - N = 2*\ceil{bitLen(q)/256} (# EVM words necessary to represent elements of Fq)

[AntoineRondelet]

Side note2: Making the constructor below generic shouldn't be really hard, but troubles may start with functions below, so that may be better to have different contracts to avoid any complexity related to generalization (and so my comment above may not apply anymore if the group name appears in the contract name)

[dtebbs]

I've added a comment at the top to explain that this all assumes alt-bn128.

Making the constructor below generic shouldn't be really hard, but troubles may start with functions below, so that may be better to have different contracts to avoid any complexity related to generalization

Yes, good idea to rename, especially once we support another curve. It will be interesting to see just how much of the contract can be made generic. But to make it properly generic without some kind of templating I think we'd probably need to pass several extra parameters (to specify the size of field and curve elements, the value of `g2, etc) and extra logic at runtime to deal with proofs and verification keys in dynamic arrays.

It would be nicer to split the verification part into it's own library. I seem to remember hit issues with this in the past, but it's working in zecale so we may be able to replicate what happens there.

[AntoineRondelet]

Name is inconsistent with argument name in verifyTx for Groth16

[dtebbs]

Renamed the groth16 version from proof to proof_data. (In the pghr13 case this function fills out an actual Proof structure, which is naturally named proof. The groth16 version is optimized to pull directy from the memory buffer.)

[AntoineRondelet]

LGTM