Fix critical issues SI84 and SI85 from Cure53

[aniampio]

This pull request fixes the following issues:

• NYM-01-009 WP5: BLS12-381 EC signature bypasses in Coconut library (Critical)
• NYM-01-014 WP5: Partial signature bypass in offline eCash (Critical)

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[jstuczyn]

I will look at it as soon as I have some time. Please do not merge it before then.