Clarify iteration behavior for causes by splitting it in two methods

[mitsuhiko]

This changes the new iter_causes method to really only include causes
and add iter_chain to iterate over the entire chain including the
outermost fail.

[matklad]

An interesting observation is that, I think, one typically uses .causes just to print the errors. So, the need for these methods might be significantly reduced if a Display impl just did the right thing from the start.

[mitsuhiko]

Display in itself can’t but i want to add a method that returns a formattable object that prints all causes. I just did not want to cram that into this release.

[mitsuhiko]

@matklad I added this as an experiment. It's based on how we render our own failures: #233

[mitsuhiko]

I'm going to merge this.