Saner items API names
Minor test updates
CursorItemIter fixes

- `pos` wasn’t used anywhere anyway

- added unwrapping
Fixes in iterator size hints

Both KeyRange and Cursor iterator actually don’t know anything about bounds (to be precise - their estimation is number of keys in database, which could be huge, so we’re ignoring them for now).

Only Item iterator can give a correct number after it was pointed to correct key. Note: pointing to key happens only after first access.