Mod operator does not work for i16 / i8 inputs

```
def main():
    i: i16; j: i16; k: i16
    i = i16(10)
    j = i16(3)
    k = i % j
    print(k)

main()
```
Gives :
```
$ lpython a.py 
semantic error: Arguments do not match for any generic procedure, _mod
 --> a.py:5:9
  |
5 |     k = i % j
  |         ^^^^^^
``` 

Should get fixed by overloading the `mod` function.