Implement base functionality

• Add Operation abstract class
  • Represents a proof operation
  • Serialize with toJSON
  • User friendly representation with toString for use with the CLI
  • Implement three operations: Prepend, Append, and SHA256
• Add Proof class
  • Represents a cryptographic proof
  • Serialize with toJSON
  • Parse with Parse.parse static method
  • Derive root value by feeding an input
  • Consideration for passing to a Prover instance (unimplemented)
  • Consideration for handling and upgrading legacy proof serialization formats
• Add ad hoc visual test: test/test.js