Deterministic ordering of generated outputs

[sdboyer]

Currently, thema's code generators do not do any additional work to ensure stable output order for generated results. When underlying libraries change their output form, such as in #107, it may result in unnecessarily noisy diffs, making it more difficult to visually assess that a particular change is safe to make.

If we decided to put in place a rule that guaranteed deterministic output ordering, we might be able to stabilize outputs somewhat. Unclear how difficult or productive this would really be, though.