Document and enforce public vs private API

[MatthewPeterKelly]

All functions and classes in the library are currently part of the public API, although many are intended to be for internal use only. We should decide what will be part of the public API (for focus on thorough documentation) and what should be part of the private API (easier to clean-up the code without worrying about breaking things with every change to a private function).

1. Decide what will be in the public API
2. Decide how to separate the private and public sections of the code
3. Implement!

[rethink-forrest]

Step 4. Profit!