Replace calculation back-end of Calculator with the GMP library

by Daniel Renninghoff for GNOME

The calculation back-end of GNOME Calculator has many limitations and is very old and buggy. In this GSoC I want to replace it completely with the GMP library, which has many benefits: a lot of calculation bugs get fixed, it allows arbitrary precision arithmetic (no 9 digit limit anymore!), and the calculations are faster. This also makes maintaining and contributing to GNOME Calculator easier.