[Le #150] Improve documentation and code organization in core files

[stanislowskij]

⚠️ Imported from legacy repository. This issue is a duplicate of https://github.com/Clojure-Intro-Course/babel-legacy/issues/150.

This is more of a long-term goal, but there has been a fair bit of talk about the messiness of comments and docstrings throughout the project. I've already started working on this a bit, but it'd be nice to have some well-defined goals. I think being more clear about how things work and organizing the code correctly would help with getting people involved in the project more quickly in the future.

• [ ] Remove unused requirements, definitions, etc. where necessary
• [ ] Lint code for style standards - e.g. line length, indentation, var names (re: #149)
• [ ] Include comments explaining some of the higher-level abstractions in core files, such as middleware, corefns and processor (although most functions in processor.clj have sufficient docstrings, some clarity in the specifics would help)
• [ ] Consider putting together a CONTRIBUTING.md to help new people maintain code organization?
• [x] Maybe include some info about the current situation with (setup-exc) in the README.md, and maybe redirect people to the nrepl-caught branch if they are looking at the (extremely dated) master branch, since this seems to be where we are doing our work now.

[stanislowskij]

Refactoring commits associated with dictionaries.clj and prettify_exception.clj should also mention #34.