Add Slicing

Add slicing operators

```Python
In [2]: s = Symbol('s', '5 * 10 * {name: string, amount: int}')

In [5]: s[0].dshape
Out[5]: dshape("10 * { name : string, amount : int32 }")

In [6]: s[:2, 3::2].name.dshape
Out[6]: dshape("2 * 4 * string")
```
should probably raise if `index.stop` is not an integer