hide lmdb::RoCursor type from consumers of Reader/Writer structs

[mykmelez]

The Readable trait in readwrite.rs currently leaks the lmdb::RoCursor type, and we generally don't want to show consumers the types from the lmdb crate dependency (preferring to encapsulate them in higher-level rkv types). We should figure out a way to hide that type from consumers of the Reader/Writer structs that implement the Readable trait.