Implement `axes` and `iterate`

[giordano]

This package currently implements only the basic AbstractArray interface, but not the optional methods such as iterate or axes, which leads to some funny (and wrong) results.

[giordano]

Mostly addressed by f1e687f, 4f90098, and fe8a413. The main remaining issue I see is that now collect(::StarWarsArray) fails (it didn't use to up to 8e41eef)

[mkitti]

axes is supposed to return

a tuple of AbstractUnitRange{<:Integer} of valid indices

[giordano]

How do you think you can do a AbstractUnitRange{<:Integer} for this indexing?

[mkitti]

I'm just quoting @mbauman here

https://discourse.julialang.org/t/offsetarrays-inbounds-and-confusion/81295/50

[mkitti]

I suppose that axes only has to return valid indices. They do not have to be in iteration order.

[jariji]

They do not have to be in iteration order.

Can't we just add that requirement to the AbstractArray docs? It seems like a complication and I'm not seeing much advantage.