Serialize to EDS

[goodmami]

Elementary Dependency Structures get a bit of usage, so pyDelphin should put more effort into getting these right. We already have a rudimentary (and incorrect) serializer, but it needs better support for QEQs and "disconnected" graphs. See the LKB's Lisp code for its head-finding routine, which is more sophisticated than the DMRS one (though I'm not suggesting the DMRS code should use it).

Since EDS are lossy, this issue is only about serializing TO EDS, not deserializing from them.