Threshold signing library based on CGGMP'21 scheme.

This library is based on the scheme described in "UC Non-Interactive, Proactive, Threshold ECDSA with Identifiable Aborts" by R. Canetti, R. Gennaro, S. Goldfeder, N. Makriyannis, and U. Peled, DOI:10.1145/3372297.3423367 (preprint is available at https://eprint.iacr.org/2021/060, and it has been revised compared to the published version).