py: Convert [u]int to mp_[u]int_t where appropriate.

Addressing issue #50.
diff --git a/py/mpz.h b/py/mpz.h
index a2d8923..9dbbbc3 100644
--- a/py/mpz.h
+++ b/py/mpz.h
@@ -88,7 +88,7 @@
 bool mpz_is_odd(const mpz_t *z);
 bool mpz_is_even(const mpz_t *z);
 
-mp_int_t mpz_cmp(const mpz_t *lhs, const mpz_t *rhs);
+int mpz_cmp(const mpz_t *lhs, const mpz_t *rhs);
 
 mpz_t *mpz_abs(const mpz_t *z);
 mpz_t *mpz_neg(const mpz_t *z);