Adding the get output function

[hecmas]

Related to #22 and #18.

Add a utility function getOutput that takes a witness and a map where the keys represent the desired output names and the values indicate the number of elements to retrieve. It returns the corresponding circuit outputs from the witness.

For example:

let output = await circuit.getOutput(witness, { a_inv: 5, b_inv: 5 });

gives back:

{
  a_inv: [
    18015910124504391758n,
    10263769591271306743n,
    17554040121178085313n,
    8619307900730399718n,
    11898114031726211586n
  ],
  b_inv: [ 0n, 0n, 0n, 0n, 0n ]
}

---

It is perfectly fine to specify fewer outputs than the total available, as well as to request fewer elements than the full output size. For example:

let output = await circuit.getOutput(witness, { a_inv: 3 });

would produce the following output:

{
  a_inv: [ 18015910124504391758n, 10263769591271306743n, 17554040121178085313n ]
}