feat: reimplement proof APIs and refactoring

[dev0x1]

Closes #36 Partially Closes #22

• reimplement proof APIs(proof_gen and proof_verify): single API and single-file implementation simplification over existing multi-file & multi-function implementation
• add KeyPair type and club key* APIs secret_key and public_key in a single module
• publicly export only the bbs::api module(types and APIs)
• new tests module for unit tests and move all existing unit tests to this new module
• unit test cases for proof* APIs
• implement integration tests for bbs::api APIs
• clean/rename errors