BLS batch verification (restructured)

[gtank]

This adds raw support for batch verification. It differs from the previous PR by being adapted to the new repo structure from https://github.com/celo-org/celo-bls-go/pull/25.

(From README.md) There are a number of codegen steps remaining before this is ready to go:

• [x] Create a new branch
• [ ] Delete the old libs
• [x] Create a PR
• [ ] The CI will now build all the libs and store them as tar.gz. artifact.
• [ ] Download the tar file and extract it in the root of the repository. This will create the libs directory.
• [ ] Run go run cmd/distribute/distribute.go . platforms/platforms.json. This will create all the repositories for the different packages.
• [ ] Run ./scripts/push_and_tag.sh TAG with a chosen TAG. This will create a tag in each of the repos and update the go.mod.
• [ ] Merge the PR, so master will be up to date.
• [ ] Run ./scripts/push_and_tag_master.sh, to push the tag to the main repository.

[gtank]

It looks like CI won't run until someone reviews it, but the CI artifacts are part of the build process. Due to a process oversight, I lack the permissions to kick it.

This is currently blocked until either someone with permissions can review it or I can fix my permissions situation.