Support outer indexing (NEP 21)

[asmeurer]

https://numpy.org/neps/nep-0021-advanced-indexing.html

[asmeurer]

I think an actionable thing to have here would be to have a function oindex that works like ndindex() except it treats the input as an outer index, and converts it into an equivalent vectorized (i.e., normal) index. This would be an extension of the numpy ix_ function, except unlike ix_, it would support arrays with dimensions other than 1, and it would support mixing non-array indices (as long as the array indices aren't separated by non-array indices. That I still want to leave unsupported/unimplemented).

[asmeurer]

The NEP has some links to implementations which could be useful. What actually would be the most useful would be an implementation that we could test against, but I think we'll just have to write a more direct behavioral test. I expect that to be the hardest part of implementing this.