distinguish goals and applications

[nikomatsakis]

The WASM blog post skill tree had a really clean look:

My eventual goal is that our skill trees should look like that. I'm not sure how close we can realistically get, but as a first step, it would be great to distinguish "goals" -- represented as the "circles" in the above diagram" -- from the "groups of tasks" -- represented as rectangles in the above.

In our current graphviz output, they render the same.

One challenge I encountered when messing with this: line wrapping.