py/mpz: Make mpz_is_zero() an inline function.
It's more efficient as an inline function, and saves code size.
diff --git a/py/mpz.h b/py/mpz.h
index 878febf..2ff404f 100644
--- a/py/mpz.h
+++ b/py/mpz.h
@@ -111,7 +111,7 @@
size_t mpz_set_from_str(mpz_t *z, const char *str, size_t len, bool neg, unsigned int base);
void mpz_set_from_bytes(mpz_t *z, bool big_endian, size_t len, const byte *buf);
-bool mpz_is_zero(const mpz_t *z);
+static inline bool mpz_is_zero(const mpz_t *z) { return z->len == 0; }
int mpz_cmp(const mpz_t *lhs, const mpz_t *rhs);
void mpz_abs_inpl(mpz_t *dest, const mpz_t *z);