iter() should iterate single element indices

[asmeurer]

For example, list(Slice(0, 3)) would return [0, 1, 2]. list(Tuple(Slice(0, 1), Slice(0, 1)) would return [(0, 0), (0, 1), (1, 0), (1, 1)]. It would raise ValueError on a Tuple with an ellipsis.

An iter would be infinite if the length of a slice is unbounded, but a shape reduced index should always give a finite iteration of exactly prod(shape) items.

[asmeurer]

This has the needs decision label because of #17.