[rocks-lib] minimise exposed API

[mrcjkb]

Currently, most of the functions and types in rocks-lib are public.
We should make anything that doesn't need to be public pub(crate).

[vhyrro]

As I'm doing the lua bridge I'm discovering which functions need/don't need to be exposed by default, so we should be able to use that as a baseline no what to make public :)