Improve show performance by only evaluating each `:render-fn` once

[mk]

Previously each node in the viewer tree would carry a viewer node and so we'd read and evaluate it multiple times. Now we build up a lookup table from hash to viewer and do it only once per unique viewer.

TODO:

• [x] dedupe viewers
• [x] convey closing parents via opts instead of viewer
• [x] Adjust tests
• [x] Fix closing parens for (concat (range 20) [#{1 2 3} :foo])