Add specification/documentation comments

[WillBAnders]

The current codebase is almost entirely undocumented, mainly relying on test cases to clarify expected behavior. This slows down both new features and refactors since they often affect these unstated assumptions, and also limit the ability for other contributors to understand the codebase as well.

This is a tracking issue covering the main parts of the repository. Documentation should be included for most functions, with a particular emphasis on the analyzer and environment (which currently have the most complexity).

• [x] Parser (including rough grammar)
• [ ] Analyzer
• [ ] Evaluator
• [ ] Environment
• [ ] Library (mainly refer to Kotlin/Java methods as needed)