Feature/prefix matching

Adds support for prefix matching.

- ol_prefix_match. This function returns a msgpack encoded list of values from keys that match a given prefix. Also adds some preparatory code for cursor iteration (`ols_next_node`).
- More tests for the new stuff

Caveats: The tests don't actually test decoding of the created data. Ideally I'd like to do this somewhere else, like in an official implementation of msgpack in python or something. I'm tired.

/cc @Hamcha @kyleterry