add checking of numeric indices to an array to see if they can reasonably be considered integers. Avoid undefined behaviour if out of bounds

jv.h

@@ -60,6 +60,7 @@ jv jv_bool(int);
 
 jv jv_number(double);
 double jv_number_value(jv);
+int jv_is_integer(jv);
 
 jv jv_array();
 jv jv_array_sized(int);