Consider supporting slicing with symbols

[arcondello]

Currently you can index one array symbol with another

A = model.constant(np.arange(5))
idxs = model.constant([0, 1, 2])
A[idxs]

and you can slice array symbols

A = model.constant(np.arange(5))
A[:3]

but you cannot include other symbols in the slice

A = model.constant(np.arange(5))
stop = model.constant(3)
A[:stop]  # throws an error

We should consider supporting this.

See also #327

[arcondello]

I just remembered that we already have ARange, see #232 which does allow this in some cases. See

A = model.constant(np.arange(5))
stop = model.integer(upper_bound=4)
A[arange(0, stop)]

We added it for this exact purpose. We could/should update ArraySymbol.__getitem__() to do it automatically. I honestly forget why we didn't just go ahead and do that at the time.

[wbernoudy]

If we want to support the common use case for this of slicing with the size of another symbol, so that you can use the slice with the original symbol as equal-size arguments to some other function, arange() will have to report its size more intelligently.

For example, this doesn't work currently because arange won't report its sizeinfo as being equal to extracted's size:

extracted = extract(condition, arr)
put(b, extracted, c[arange(extracted.size())])