Consider supporting slicing with symbols

Currently you can index one array symbol with another
```python
A = model.constant(np.arange(5))
idxs = model.constant([0, 1, 2])
A[idxs]
```
and you can slice array symbols
```python
A = model.constant(np.arange(5))
A[:3]
```
but you cannot include other symbols in the slice
```python
A = model.constant(np.arange(5))
stop = model.constant(3)
A[:stop] # throws an error
```

We should consider supporting this.

See also https://github.com/dwavesystems/dwave-optimization/issues/327