Abstract away witness generation within noir_js

[signorecello]

Part of the first noir_js devex review.

With the current versions, we need to call two methods in order to generate a witness:

const solvedWitness = await generateWitness(circuit, input);
const witness = witnessMapToUint8Array(solvedWitness);

this also means that devs need to import and manage versioning for noir_js, acvm_js, and optionally noir_wasm, reducing the usefulness of noir_js in the first place

would be worth to try making them only one, and abstracted within the noir_js main export (class, object, whatever it is). This avoids passing the circuit twice, as it can be stored within the Noir class. Suggestion:

const noir = new Noir(circuit) // here goes the circuit, is stored in the class
const witness = await noir.generateWitness(input) // just need to pass the input

[signorecello]

I wonder if the generateWitness method we have in noir-starter could be useful

Didn't yet check acvm_js library but it seems to be an amazing lib we could just use in this method instead of the low-brain Map.set we have there 😄 Actually the reason I didn't try it in noir-starter is because it will be refactored pretty soon with noir_js anyway