v2020.12.19

Major changes includes:

• added secp256k1 point multiplication based on efficient endomorphism
• fixed ssa batch verification functions' logic
• enforced snake_case variable naming convention, e.g. 'script_pub_key', etc.
• added BIP32KeyOrigin, BIP32KeyPath, and BIP32KeyPaths
• adopted str instead of bytes as default type for BIP32 keys, bms sigs, PSBTs, addresses, and WIFs; base58 and bech32 encodings keep returning bytes, like base64
• cleaned up and refactored all dataclasses, now using serialize/deserialized and possibly b58encode/b58decode, b64encode/b64decode, etc.
• renamed TxIn.witness as TxIn.script_witness
• fixed Witness management in TxIn and Tx, including equality operator
• consolidated sig_hash code into sig_hash module
• added more script_pub_key functions: assert_p2pkh, is_p2pkh, etc.