Test cases for workflows (with toy functions, for now)

[amakelov]

For this issue we want to

• have more test cases... :)
• get an idea of what we're dealing with in terms of the "synthetic" properties of workflows:
  • the composition logic (how are the functions composed, e.g. nested loops as @nschiefer described to me);
  • the "quantitative" dimensions of workflows, like how many function calls we have ("total number of nodes in the computational graph"), and the size of the longest sequence of chained function calls (something like the "diameter of the computational graph");
  • the interfaces of the functions involved, especially if there's going to be something funky going on.

Other aspects of this will be very simple (in-memory storages for now, toy functions).

I'm working on providing a rich enough environment to do this in (most importantly, enabling superops in the two flavors we discussed).