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

The aforementioned schemes are built using the following primitives:

1. Set Commitment Scheme
2. Mercurial signatures
3. Structure-Preserving Signatures on Equivalence Classes (SPE-EQ) which are realized using Mercurial signatures
4. Structure Preserving Signatures on Equivalence Classes on Updatable Commitments (SPSEQ-UC) which is also realized using Mercurial signatures.