New Body Plan: Make all bodies snake-like (temporarily)
New Body Plan: Add one gene to chromosomes

To make space for new control flow genes.
New Body Plan: Extract constants for genes in embryology

To get one place to make sense of it all instead of having scattered
meaning over the whole package.
New Body Plan: Renumber genes in chromosome
New Body Plan: Add instructions for Continue, Branch and Stop
New Body Plan: Add better branching tests
New Body Plan: Add instruction for Mirroring
New Body Plan: Add test for recursive mirroring
New Body Plan: Integrate new body plan with embryology
New Body Plan: Tweak parameters

To make the bodies more interesting, and bring back some performance now
that the narjillos tend to have much more complex bodies.