Closes #44

Null attributes are attributes with bigint value 0, meaning that the issuer skipped that attribute. Instead of aborting the session, the irmaclient now allows disclosures of null attributes, when the verifier requests it. This allows the irmaclient to prove that a particular attribute was not issued to it, and allows verifiers to request optional attributes alongside other attributes.

fix: both irmaserver and irmaclient now early reject session requests referring nonexisting identifiers

feat: Breaking change: support conjunctions of disjunctions of conjunctions when requesting attributes in any of the three supported session types

Instead of requesting several attributes, one can now request several groups of attributes. Within each such group, attributes belonging to the same credential type always come from the same credential instance. This affects all three session types, as in each of them attributes can be disclosed.

chore: Add unit_tests build flag to filter out integration tests. Moved helper methods from session_test to main_test.

Co-authored-by: Confiks <confiks@scriptbase.org>

Co-authored-by: Confiks <confiks@scriptbase.org>

Co-authored-by: Confiks <confiks@scriptbase.org>

Co-authored-by: Confiks <confiks@scriptbase.org>

Co-authored-by: Confiks <confiks@scriptbase.org>