Use gmp_[f]printf in many places

Hi Paul,

I found this to be a small improvement to the readability of eval.c, but I'm also aware each change takes time to review and has a chance to introduce bugs. Let me know if I should not make this type of cleanups.

At work every piece of code is code reviewed by two people. For changes here I try my test to test them with make check and additional manual spot testing. Should I be using this merge request flow + code review for all of my changes or is it more appropriate for larger functionality changes?