[native] restructure functions to be exposed across FFI boundary

Summary:
CXX doesn't have as much support for methods as I thought, so I'm changing the methods to normal functions that take an additional client param. We use a Box pointer because opaque Rust types can't be passed directly. I also removed all opaque Rust types from the return values of these functions.

Depends on D4935

Test Plan: cargo build, this is just a slight refactor even though it looks big

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D4936