FF/Group Tests

A series of sanity checks for implementors of the ff/group APIs. Implementors are assumed to be of a non-trivial size. These tests do not attempt to check if constant time implementations are used.

This library was [audited by Cypher Stack in March 2023](../../audits/Cypher Stack crypto March 2023/Audit.pdf), culminating in commit 669d2dbffc.