Make Broadcasted iterable and more indexable

[mbauman]

Defines iteration and some related traits as well as a few functions to make indexing a bit friendlier.

[mbauman]

Any comments here? I feel like this should be largely unobjectionable, but I do want to point to my comment here #19198 (comment): defining iteration pretty much locks us into the status quo on the evaluation strategy.

Travis 32bit failure was a bunch of ProcessExitedException()s — maybe OOMs? Appveyor looks like timeouts.

[andyferris]

FWIW I agree more-or-less completely with #19198 (comment), especially

While it feels like we should be able to refactor Broadcasted to work purely based upon iteration alone (and not indexing), I think it'd be really hard and possibly impossible for some combinations of iterable arguments. Doing so performantly is even worse.

[StefanKarpinski]

I think the breakdown of map is for iterable things and broadcast is for indexable things furthers that position as well, so 👍. Another potential use for this is lazy construction of a broadcasted object which you could then do random access indexing into if you don't actually want to materialize the entire broadcasted array, which seems like it could be very handy sometimes, as well as intuitive. That seems at odds with an iteration-based conception of broadcasted objects.

[mbauman]

Great. Let's re-run CI now that I think things are back in working order.

[mbauman]

Travis Mac failed with a GitError(Code:ERROR, Class:Net, curl error: Could not resolve host: github.com.