DX: Standardise taxonomy load/save/delete functions on objects

Here's a patch.

I'm doing a cast to object in case taxonomy_term_save() is passed an array. I'd like to get rid of this, but this means rewriting 90% of the existing taxonomy term tests (same for vocabulary which needs the same treatment). I can do that here, but ideally it belongs in a different issue. Also drupal_write_record() casts to array internally as well, so I dunno if we might want to leave it as a helper along those lines.

Following pwolanin's improvements to #329140: Make vocabulary load/insert/update/save like terms, here's the same for taxonomy terms. taxonomy_term_save() can no longer be used to delete a term - apparently this wasn't used anywhere in core, but still...

I missed a couple of cast to objects, and one typo made it through. Found this when saving new terms from the node form - so we need a test for that, and taxonomy.test is a real mess in general.

Here it is with a test :)

added code comments and whitespace changes.

and fixed a typo that appears to have become unfixed in one of the revisions.

Back to needs work for docs.

Also byproduct of dbtng conversion was this bug (also not tested, but now it is) #332145: UNSTABLE-3 blocker: taxonomy_form_term_submit passes empty string as parent [dbtng conversion regression].

Added documentation http://drupal.org/node/224333#term_functions