Programmatic term updates destroy hierarchy

It should be possible to do:
$term = taxonomy_term_load($tid);
taxonomy_term_save($term);

Same as you can with any other entity in core. However our crappy hierarchy handling breaks on this.

For one, unless you explicitly load term->parent it'll get wiped. Additionally we delete then insert on both inserts and updates when it's only needed for inserts.

i'd prefer it we could load $parents in taxonomy_term_load() but that's much more of an API change. The only one here is that if you wanted to kill parents in a programmatic update you'd have to explicitly set parent to 0.