Add tests for the GMP implementation of the RQFT

Disable frobenius_int tests to make everything compile and fix bu in
split where the condition in the while-loop was reversed for some
reason.
Fix typo
Fix bug in mult_mod

Calling mult_mod would leave *res1 untouched if the function
was called with d==0.
Make sure powm only changes its output parameters
Remove two unnecessary % operations
Add const declarations
Add new tests for the GMP implementation