We started with a string because we thought it could maybe provide more
flexibility, though really, an int makes much more sense, especially on
all the platforms etebase runs nowadays.
Filesystems don't handle massive directories too well, so better to split.
Using the prefix of the chunk gives us a maximum of 64 * 64 = 4096 entries
in the main directory.
Each collection now has an item and the item's UID is the collections
UID. This lets us manipulate collections just like items, and as part of
transactions. This is significant because it lets us change them as part
of transactions!
