Allowing `None` for indexing to add an axis

[jakirkham]

In many array libraries today, one can use None when indexing to add a singleton dimension like np.newaxis. Would be good to support in the standard as well.

[rgommers]

Thanks @jakirkham. I think this one fell through the cracks so far. x[:, None, :] is equivalent to expand_dims(x, axis=1), the latter is in the standard so at least there's an easy alternative.

I just tested and it looks like every library supports indexing with None with consistent behavior. So we should just add this I think.

[asmeurer]

We should probably also add newaxis = None to the namespace.

[kgryte]

I've opened #408 which adds support for expanding dimensions via None.