Finish adding CHKInventory as a permutation in per_inventory.
Get the Read tests working. Change the apply_delta tests to be create_by_apply_delta and implement that functionality on the regular Inventory class. (just does a .copy() and then apply delta on the result.)
This also required fixing a lot of the tests that never asserted the objects were valid. Namely: InventoryFile('file-id', 'name', 'parent-id') does not generate a fully valid file object, as it has no text_size or text_sha1.
The filter() tests are finally failing, which means I have a chance to actually fix things via tests.