Within a session request, inner conjunctions requesting specific attribute values
used to be serialized to a map. JSON and Go maps are however unordered, and the
order matters: if the requested attributes are received in a different order then
the session will fail. Inner conjunctions using values are now serialized using
the default Go serializer, resulting in a list of requested attributes with their values.

Fixes #43

Within an inner conjunction of a session request, distinct attributes that come
from the same credential type must always be adjacent. This is now enforced in
the irmaclient and server packages when receiving session requests.

When passing --server to irma session, it would erroneously complain about
--url also being set.

fix: irma server now shows error message when invalid requestor json is passed using flag or env var

Fixes #38

Fixes #37

In particular, combined issuance sessions in which 2 or more attributes are
requested from one particular credential type, due to a miscalculation during
verification of the attributes.

Fixes #36

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.