Redesign CaDiCaL prooftracer API

Redesign the CaDiCaL prooftracer API in a similar fashion to the solver API in general.
Attaching a proof tracer should wrap the solver type where all methods are still available.
This allows for accessing the proof tracer with methods along the lines of `proof_tracer_ref()` and `proof_tracer_mut()` while preserving exact type information.