now supporting signed credentials of keypairs, shorted issuer
verification keys and a simplier flow matchin the petition

keypair alone means credential
issuer means the credential signer

hurray!!!

also covered by tests, coconut tests re-activated in CI

equivalent to mod_inverse in py/petlib big numbers, computes the
inverse mod such that self * res == 1 mod m

also made more modular the selection of elements

still not returning checksum correctly, to be fixed (tests pass)

up until android v5 there are several bugs in the NDK preventing
backward compatibility. At the time of writing this means a loss in
~10% of old devices according to this table
https://developer.android.com/about/dashboards/?hl=en-419

see https://android.googlesource.com/platform/development/+/b603563

using the private key of petition owner as signature secret

includes refactoring of schemas for zencode ccoconut

still blocks at the miller loop in verify proofs

this allows to simplify and duplicate zencode directives to cover
different executions for different contexts

and various other improvements on zencode execution and debugging

now much easier to define zencode schemes: defined globals IN/ACK/OUT
for processing, evolved schema checking for complex structures and
added possibility to import/export according to schema