GM17 verifier gadget for libsnark

[PossiblyHarry]

Fixes: #2
Fixes: #4

This implements the GM17 verifier and G2 point addition gadgets, with support for mnt[46]753

It is a branch off of #1

Constraints, mnt4, verifies mnt6:

• Number of constraints in [convert 3 input to bits]: 894
• Number of constraints in [validate proof]: 21
• Number of constraints in [validate verification key]: 63
• Number of constraints in [verification key constraints]: 19372
• Number of constraints in [proof verifier constraints]: 58029
  number of constraints for verifier: 78379 (verifier is implemented in mnt4 constraints and verifies mnt6 proofs))

Constraints, mnt6 verifies mnt4:

• Number of constraints in [convert 3 input to bits]: 894
• Number of constraints in [validate proof]: 13
• Number of constraints in [validate verification key]: 39
• Number of constraints in [verification key constraints]: 10024
• Number of constraints in [proof verifier constraints]: 34069
  number of constraints for verifier: 45039 (verifier is implemented in mnt6 constraints and verifies mnt4 proofs))

[PossiblyHarry]

Note, I changed the submodule URLs in .gitmodules which may cause problems.

-	url = git@github.com:EYBlockchain/zk-swap-libff.git
+	url = git://github.com:EYBlockchain/zk-swap-libff.git

-	url = https://github.com/scipr-lab/libfqfft.git
+	url = git://github.com/EYBlockchain/zk-swap-libfqfft.git

I need an access token with 2-fa enabled account to commit to HTTPs urls, so switched to git, but github actions cannot checkout the public repo using git:// urls, so will need to switch back to https://?