[circuits] [crypto] review the signing and signature verification code

[weijiekoh]

Some of the cryptographic code (in crypto/) is custom-written and could do with a review.

[ctrlc03]

The signature code is now handled by zk-kit eddsa-poseidon package and the code was audited as part of semaphore v4 audit: https://semaphore.pse.dev/Semaphore_4.0.0_Audit.pdf