Delegatable credentials

Provides 2 variations:

1. Ad-hoc delegation where credential owner chooses a set of issuer public keys and proves that the credential was issued by one of the key. Based on the paper Protego: A Credential Scheme for Permissioned Blockchains. Code
2. Here there is a root issuer which can issue a credential to anyone with the permission to reissue the credential with or without additional attributes. Based on the paper Practical Delegatable Anonymous Credentials From Equivalence Class Signatures. Code