py/mpz: Make mpz_is_zero() an inline function.

It's more efficient as an inline function, and saves code size.

py/mpz.c

@@ -939,10 +939,6 @@ void mpz_set_from_bytes(mpz_t *z, bool big_endian, size_t len, const byte *buf)
     z->len = mpn_remove_trailing_zeros(z->dig, z->dig + z->len);
 }
 
-bool mpz_is_zero(const mpz_t *z) {
-    return z->len == 0;
-}
-
 #if 0
 these functions are unused