Update implementation and tests to latest draft

[tmarkovski]

The BBS spec has been updated with a change that optimizes the signature size.
The implementation and test fixtures need to be updated.

Removed Q1, s, and corresponding values, following the first proposed optimization in the EuroCrypt 2023 Revisiting BBS paper (and then renamed Q2 as Q1). Proof generation and verification modified by @andrewwhitehead to maintain compatibility with spec extensions; changes reviewed by the paper authors.