Replace GMP with MPIR

Summary:
MPIR is written in ANSI C and assembly language and runs on many
platforms (including Linux, Mac OS X and Windows on common hardware
configurations). It's a snap-in replacement for GMP.

Test Plan: compile, run tests, make sure libgmp is not referenced anymore, build KMyMoney using it, check that it compiles on KDE CI

Reviewers: wojnilowicz, bcooksley, habacker

Reviewed By: wojnilowicz

Differential Revision: https://phabricator.kde.org/D11212