- fetch sfld and csum when fetching notes, to make it cheaper
to write them back out unmodified
- make `fields` private, and access it via accessors, so we can
still catch when fields have been mutated without calling
prepare_for_update()
- fix python importing code passing a string in as the checksum
Originally removed because some users were adding cloze deletions to
it, but removing it just replaced that problem with a new problem where
users add the field but don't add it to their card templates.
