Update BatchCompat VRF to audited version of libsodium (and latest draft version)

[iquerejeta]

We got a successful audit by Kudelski for the batch verification implementation of VRF proofs, which is implemented in this branch. This PR updates the haskell bindings, and the cbits, to the new design using the audited code. Similarly, we adapt the code in cbits/private to the code available in libsodium/stable v1.0.18, instead of some version of master we had previously. By not being based in master not only we use stable code, but we also remove the following warning we had when compiling:

# warning *** This is unstable, untested, development code.
# warning It might not compile. It might not work as expected.
# warning It might be totally insecure.
# warning Do not use this except if you are planning to contribute code.
# warning Use releases available at https://download.libsodium.org/libsodium/releases/ instead.
# warning Alternatively, use the "stable" branch in the git repository.

Similarly, we re-activate the BatchCompat module, which was deactivated in the past due to a lack of external audit.

This closes #337

[tdammers]

Haskell changes look trivial, have not looked at C code.

[lehins]

I am sure the audit by Kudelski was pretty thorough, cause I am not gonna pretend like I reviewed and understood what is going on in the C implementation of the VRF.

But I did read through a good portion of the PR and commented on obvious things that jumped out at me. Didn't find anything serious, just minor suggestions and questions.

Also, I think it would make sense to squash the first three commits in the PR

[iquerejeta]

@lehins , I have applied the suggested changes. We are now only missing that the update of libsodium is merged into iohk-nix. Just want to check that we are all good on this side to merge before pushing for that merge (just in case we need a final touch on the libsodium fork) 👍

[lehins]

Just two tiny related comments, otherwise, yeah, I think it is ready to go.

[iquerejeta]

Waiting for nix PR to be merged before merging this.