Build script model using Mori

[dethe]

Mori is a library extracted from ClojureScript which supports immutable collections efficiently. This can make undo/redo super easy to support and having our model extracted from the DOM can simplify validation, speed up runtime, etc.

Mori plays nicely with React if we decide to go that way for the view.

[dethe]

More generally, this should be an experimental branch to make all objects immutable. In other words, we never modify an object in place, but always return the new, modified object. So instead of step blocks for mutations, every object change becomes an expression.