REF: make array_to_datetime single-pass
Or more specifically, remove the need for falling back to datetime_to_datetime64 which does a second pass.
cc @mroeschke  This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. can you rebase > can you rebase

yah will circle back before too long, need to address some other comments above.  pretty focused on MultiIndex bugs ATM. mothballing to clear the queue, will revisit