Abstract proof hints writer

[theo25]

This PR refactors the process of outputting proof hints to file so that it is handled by an abstract proof_trace_writer. We then implement a subclass of the proof_trace_writer class, namely proof_trace_file_writer, that outputs the proof hints to a binary file.

We do that so that we can easily add other methods of outputting proof hints in the future, such as a shared memory / ringbuffer based writer that will work in tandem with the shared memory proof hints parser that was recently merged.

The PR can be easily reviewed commit by commit.

• the first 3 commits do some preparatory refactoring to normalize naming conventions that will make the big refactoring more straightforward.
• the 4th commit replaces the writer * argument of proof trace output functions with a FILE * argument, since this is the only part of the writer class that is used when it comes to hint generation.
• the 5th commit refactors all FILE * pointers related to hint generation into pointers to a placeholder class which simply wraps said FILE *. It also does some function renaming to better reflect that these functions participate in the hint generation output process.
• the 6th commit replaces the placeholder class with the abstract proof_trace_writer class and adds a concrete subclass for writing into a file, namely proof_trace_file_writer.

[theo25]

@Baltoli I don't know if you will have the time to review this one so I didn't ask for a review. If you do have the time, your comments are always welcome and appreciated!