Create a JNI wrapper for Rust's BTreeMap

[twitu]

Originally posted by @kirillt in #61 (comment)

Currently, the FileStorage JNI bindings perform an expensive conversion to return the whole key-value mapping. However, it can be made more efficient by keeping the BTreeMap on the rust side and exposing functions on it that mimic the relevant operations of a Mapping on the Java side.

Note: We only want to expose read only operations because any write related operations should go through the FileStorage functions.

So methods for getting a value for a key and iterating key value pairs will be a good starting point.