Add doc comments & tests. Some refactoring

[na2hiro]

I'm interested in using (and potentially extending) this library and read the codebase to understand how it works. Along the way,

• I added comments for what was not clear to me as well as adding doc comments for structs and methods which look important. (1st commit)
• I added test cases for some uncovered code. (1st commit)
• I refactored and optimized (hopefully) some run time code (second commit).

Please be patient if I do stuff wrong: I'm not very fluent in Rust. And thanks for the great library!

[na2hiro]

Thank you for adding doc comments, tests, and refactoring! It helps a lot. I have submitted a change request to na2hiro#1 regarding fmt. Please check it out. I wrote the answer about Hash in the comment, is this OK?

Thanks for checking this! That looks good and I merged it to the PR.

That makes sense, I removed that TODO comment from code base. Also applied the suggested grammar fixes. Could you check this now? Thanks!