Allow for passing/reusing existing WASM instance

[martyall]

NOTE: This PR looks worse than it is -- the diff misrepresents the changes.

Changes

• factor out createWASMInstance function to build the WASM instance from source code
• change witness_calculator.builder to check if being passed wasm binary or existing wasm instance

The changes are non-breaking. In fact the code path is exactly the same if you pass in WASM source code to the constructor.

Other Context

I'm working on a project which produces a WASM binary that is compatible with the circom witness calculating binary. The standards for serializing witness files, r1cs, keys etc was documented, and it was easy enough to reverse engineer the necessary API for the witness solver.

However, my WASM binary is compiled via Haskell's GHC WASM backend which requires the wasi_snapshot_preview_1 apis to be included in the instance. This PR allows me to do that with minimal (non-breaking) changes. Here is an example project which does this:

https://github.com/l-adic/factors/blob/main/app/Prover/Prove.js#L62

[OBrezhniev]

Snarkjs tests fail with these changes - witness calc is not working. See comments for changes needed to fix this.

[OBrezhniev]

These checks should be inside createWASMInstance and version object returned together with the instance (new wrapper object for wasm instance + version is needed)

[OBrezhniev]

Also variable wc is not declared in this function, but is assigned later in the code AND is used inside createWASMInstance. So maybe it would be easier to move all the rest of the code including instantiation of WitnessCalculatorCircom2/1 to createWASMInstance too (and rename the function to createWitnessCalculatorInstance)

[martyall]

you're totally right, I must have been really sleep deprived when I reworked this 🤦‍♂️

really the only change that's required here is

export default async function builder(code, options) {
    let instance;
    let wc;

    // Only circom 2 implements version lookup through exports in the WASM
    // We default to `1` and update if we see the `getVersion` export (major version)
    // These are updated after the instance is instantiated, assuming the functions are available
    let majorVersion = 1;
    // After Circom 2.0.7, Blaine added exported functions for getting minor and patch versions
    let minorVersion = 0;
    // If we can't lookup the patch version, assume the lowest
    let patchVersion = 0;

    if (code instanceof WebAssembly.Instance) {
        instance = code;
    } else {
        instance = //same definition as before
    }
    
    //same as before

The new diff looks bad because of new indentation, but this is now the only change