fix and speed up n_is_prime on 32-bit
fix and speed up n_is_prime on 32-bit

dsroche/flint2,fredrik-johansson/flint2,jpflori/flint2,wbhart/flint2,dsroche/flint2,jpflori/flint2,dsroche/flint2,wbhart/flint2,wbhart/flint2,jpflori/flint2,jpflori/flint2,dsroche/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2