Cursor::iter_from panics when database has duplicate keys

[danburkert]

@tarcieri reported this panic, which is coming from the unwrap call. It's not clear how/what is going on here, except that perhaps iter_from should return a Result. Further clarification on in exactly what situations this happens would be good to add to the documentation. Reproducible test case: tarcieri/ithos@6d15568#commitcomment-18461783

[tarcieri]

Aah, duplicate keys may be why the tests for #7 are failing (they're expecting an Error::NotFound which isn't happening)