[merge] Minor cleanup
[merge] Minor
[merge] Move to per-class mergeMap
[merge] Move tables out of merger object
[merge] Rename
[merge] Use NotImplemented as a singleton meaning "doesn't have"

And cleanup recalculate, so we don't accidentally mess something
that is NOT recalculated.
[merge] Port GDEF to mergeMap

There's some code duplication in mergeObjects that I'll refactor
later.