Cursor::iterFrom: Panics on empty databases

[reivilibre]

lmdb-rs/src/cursor.rs

Line 79 in 90e8880

self.get(None, None, ffi::MDB_FIRST).unwrap();

The above line causes a panic in the event of an empty database.
Given that the MDB_SET_RANGE: 'Position at first key greater than or equal to specified key.' option is used, it seems like it might also cause a panic if you ask for a key which is greater than any key stored.

I think that causing a panic here isn't intuitive.
Options I see:

• Return an empty iterator instead
• (breaking) Make iter_from return a Result<Iter<'txn>, lmdb::error::Error> instead.

Would a PR with one of options be OK?