Initial prototype of outputting scope tree as DOT graph

[mpilquist]

Lots to do still but opening this as a draft for now. I'll update PR description as we get closer to something mergable.

The main idea right now is to drop a Pull.debugScopes(..) call anywhere you want to inspect the scope tree. Doing so will dump a DOT graph to standard out. There's some customizability of the graph -- e.g., Pull.debugScopes(_.withSimpleResourceDetails.toDot).

For now, I've been copying the DOT output to my clipboard and then running pbpaste | dot -Tpng | ~/bin/imgcat in an iTerm2 window (where imgcat is from https://iterm2.com/utilities/imgcat).