Expressing lifetime of C buffers in an FFI binding iterator

[tomjakubowski]

@jsgf

This works fine for get/put/delete operations, but rocksdb also supports iteration, and naturally I want to map this to Rust iterators. Rocksdb iterators maintain a cursor, and you can get a reference to the current key/value for the cursor, but the catch is that those references become invalid as soon as you move the cursor.

Unfortunately this kind of iterator isn't supported by the Rust Iterator trait. See Returning borrowed values from an iterator Reddit - Dive into anything and Reddit - Dive into anything for previous discussion.