feature/GI-23: Adaptor ECC Schnorr signatures

[qubits4all]

GI-23: Adaptor ECC Schnorr signatures

• Supports generation of adaptor ECC Schnorr pre-signatures (a.k.a. verifiable encrypted signatures).
• Provides the assoc. pVerify(...) function, which verifies a pre-signature given the public key, public adaptor tweak key, and the message that was signed.
• Provides the assoc. Adapt(...) function, which converts an adaptor "pre-signature" into a full ECC Schnorr signature.
• Also supports the Extract(...) function, which extracts the private adaptor tweak, given a pre-signature and associated full ECC Schnorr signature.