Integrate `Coconut` - an Attribute-Based Credential Scheme with Threshold Issuance based on the modified Pointcheval-Sanders signature

[olegnn]

• Move the Coconut implementation (based on the modified Pointcheval-Sanders signature) into the docknetwork/crypto
• Integrate signature's NIZK (SignaturePoK) into the composite proof system
• Restore old benchmarks and add new benchmarks for the signature and proof of knowledge
• Implement tests for this integration based on the tests for BBS+ signature, use PedersenCommitment instead of MessagesPoK as messages NIZK
• Misc tweaks