Removed dep on getprime_r.h

Use mpz_primorial_ui, and mpz_nextprime which is fast for small values and faster for large intervals than iterating all primes up to a large number (see first additional tests). This also simplifies the logic a tiny bit. I have another merge which adds more validity checks and documentation on eval coming soon.

Additional tests time echo '(1000001000#1000000000+2)/283' | ./ecm -sigma 0:18 1e4 time echo '(100#+1)/2336993' | ./ecm -sigma 0:18 1e3

There are good tests to help validate this. I add factors to some of tests to verify the exact number and not just a number with any divisor.