Consider using ndindex for canonicalizing indexes

[tomwhite]

The logic in index for manipulating an index selection is fairly complicated, particularly the part that removes any ellipsis. (See #400, #401, #402.)

We could use ndindex to help with this - the following shows how easy it is to remove an ellipsis:

idx = ndindex.ndindex(key)
key = idx.expand(x.shape).raw

The library is well tested with Hypothesis tests, but it would be another (pure Python) dependency.