compare_distance predicate, and more

Hello! I am working on my library for geometry algorithms and recently I realized that for some implementations I would need more exact predicates, other than just orient and incircle. One example of the predicate that I need and could be very useful in general is "compare_distance(a, b, c)", which compares distances a-to-c and b-to-c, and returns value below 0 if a-to-c is smaller, exactly 0 if distances are the same and value above zero if b-to-c is smaller.

I am willing to to try to implement it, I think I more or less have an idea what is happening in the code in the library. The question is, are you open for adding more predicates to this library, or is it limited only to those that are implemented in original `predicates.c` and have proofs in the paper?

Overall very useful library! Thanks