Implement simplified version of structure-preserving signature scheme by Groth (ASIACRYPT 2015)

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• simplified version: Camenisch, J., Drijvers, M., Dubovitskaya, M.: Practical UC-secure delegatable credentials with attributes and their application to blockchain. ACM CCS 2017. pp. 683–699. ACM Press
• Original version for signing matrices: Groth, J.: Efficient fully structure-preserving signatures for large messages. ASIACRYPT 2015, Part I. LNCS, vol. 9452, pp. 239–259. Springer, Heidelberg

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• [x] change mapToPlaintext

@JanBobolz takes a look.

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• [x] add Type 1
• [x] add Type 2