Negative longs are treated as unsigned

[JibbSmart]

If I give mpz_t a negative long in the constructor, I get a positive mpz_t.
For now, using int instead has been fine.

Repro is quite simple:
var val = new mpz_t((long)-1);
Inspecting in the debugger gives: {18446744073709551615}

Thanks for your great work!