Reduce array indices to slices

[asmeurer]

I'm not sure if it's a good idea, but we could make reduce give a slice for an array index if it is equal. It isn't the same for arrays in tuples, so some care is required there. This also would change the index from a fancy to non-fancy index (the latter creates a view in NumPy), so I'm also not sure if it's actually a good idea.