[circuits] a quadratic vote tally circuit which supports negative votes

[weijiekoh]

Suggestions on how to proceed with this task:

1. Clone the repository and try out the build and test steps: https://github.com/barryWhiteHat/maci/blob/master/README.md

2. Read the spec and focus on the processMessage contract functionn updateStateTree circuit, and quadratic vote tally circuit.

3. Modify the spec with the solution for negative votes

4. Modify the contract, circuits, and tests. The tests are:

   • circuits/ts/__tests__/BatchUpdateStateTree.test.ts
   • circuits/ts/__tests__/UpdateStateTree.test.ts
   • circuits/ts/__tests__/QuadVoteTally.test.ts
   • contracts/ts/__tests__/Maci.test.ts

5. Don't worry about Maci.test.ts at the start. It's perfectly fine to update it last.

6. Get familiar with the way that CircleCI performs a trusted setup whenever we modify a circuit, and take advantage of it to save time. You can download the circuit, proving keys, verification keys, and verifier contracts from CircleCI artifacts. See also: https://github.com/barryWhiteHat/maci/blob/master/.circleci/config.yml#L46

[ChihChengLiang]

Close this and renew with #109 for better motivation and design approach.