Provide a nice show method for Model

[davidanthoff]

We used to have that, now it seems to be gone. I think @rjplevin mentioned that the composite branch has it again, but it would be good to have something for the workshop.

[rjplevin]

I could back-port this from the composite if desired. The truth is that the output is still quite unwieldy for non-trivial models. The show API allows you to set a :compact attribute in the IO object that the show method can test for to trim its output, but I haven't done that yet.

Is this a priority for the workshop?

[davidanthoff]

I think we need something nice for the workshop. It could be just a static string a la Mimi Model or something like that for starters. Right now it just outputs this wall of internal details, very early in any interactive demo that is really not great.