SecretKey without S2K specifier

• Updated OpenPGP.SecretKeyPacket to support unencrypted keys (no S2K specifier given, S2K usage byte of 0).
• Introduced OpenPGP.Util.checksum/1 and refactored related codebase.

Better error reporting

Address infosec code review findings by IOActive:

• BILL-LIB01
• BILL-LIB02
• BILL-LIB03

Changes summary

• Better error message for:
  • OpenPGP.Util.public_key_algo_tuple/1
  • OpenPGP.Util.sym_algo_tuple/1
  • OpenPGP.Util.sym_algo_cipher_block_size/1
  • OpenPGP.Util.sym_algo_key_size/1
• Updated Elixir supported version to ~>1.14

Encoding and Encrypting for Elgamal pub-key and AES sym. key

• Introduced OpenPGP.Encode protocol with .encode/1,2 and .tag/1.
• Add OpenPGP.Encode protocol implementation for:
  • OpenPGP.PublicKeyEncryptedSessionKeyPacket
  • OpenPGP.IntegrityProtectedDataPacket
  • OpenPGP.LiteralDataPacket
  • OpenPGP.Packet
  • OpenPGP.Packet.PacketTag
  • OpenPGP.Packet.BodyChunk
• Introduced OpenPGP.Encrypt protocol with .encrypt/1,2.
• Add OpenPGP.Encrypt protocol implementation for:
  • OpenPGP.PublicKeyEncryptedSessionKeyPacket with Elgamal (Public-Key algo 16).
  • OpenPGP.IntegrityProtectedDataPacket with AES-128, AES-192, AES-256 (Sym.algo 7,8,9).
• Added OpenPGP.encode_packet/1 that delegate to OpenPGP.Encode protocol.
• Added OpenPGP.encrypt_packet/1 that delegate to OpenPGP.Encrypt protocol.
• Add ElGamal algorithm support to OpenPGP.PublicKeyPacket.decode/1.
• Introduced OpenPGP.ModificationDetectionCodePacket.
• Introduced OpenPGP.Util.PKCS1 with PKCS#1 block encoding EME-PKCS1-v1_5.
• Refactored OpenPGP.Util.encode_mpi/1 and added exception for too long big-endian numbers (>65535 octets).