Create immutable functional style dart model objects based on JSON and JSON schemas!

Flutter is opinionated about using a functional programming model that includes immutable data objects. I've found that we tend to ignore that whole immutable thing when dealing with JSON data.

Projects can be loose when handling incoming JSON data and how it is accessed, sprinkling string-style map keys across the codebase. We can reduce that tendency by providing models to eliminate the string keys and mutable models to stop people from directly manipulating maps of strings.