AvroMapNode contains all information about the map and its current
location.
Started code to process maps.

Avro is expecting a null instead of a new element.  The only logical
explanation is that the "segment" element isn't processed, so while it
is expected to be there, it isn't.

However, this should be an array of UNION so it should be okay if it
doesn't exist.
Handles case where there is only one MAP among sibling tags (actually,
prevents there from being multiple), and only one MAP instance in the
ARRAY.

Need to support both of these, the former of MAP of UNION of RECORD, the
latter with updating the parent AvroRecordInfo whenever a MAP_START is
encountered.
Dummy mistake.