Release 0.8.2

- Update to latest libsecp256k1-zkp - Expose Sign-to-Contract functions - Expose Anti-Key-Exfiltration ("Anti-Exfil") functions - Expose base64 functions - Allow callers to provide secp contexts to make the library thread-safe Note: This release is not binary compatible with 0.8.1. See https://github.com/ElementsProject/libwally-core/blob/master/CHANGES.md for details.