Implement Schnorr signature scheme in STM

[jpraynaud]

Why

We need to implement a Schnorr signature scheme in the STM library to support the creation of SNARK Proofs.

What

Create a new module in the STM library (e.g. schnorr_signature) and implement the necessary primitives to create the individual signatures.

How

• Create a new schnorr_signature module
• Gate the module behind a future_snark feature
• Implement the primitives for creating a Schnorr signature:
  • Signature
  • Signing Key
  • Verification Key
  • Long Signature & batch verification
• Implement unit tests
• Implement benchmarks
• Update documentation
• TBD