Missing documents/comments for public items

[qinsoon]

For documentation, a first step we can do is to add missing documents for our public types and functions. We can enforce documents with rustdoc's missing_docs lints, which requires all public items to be documented. Once #241 is done, we should then only expose a limited number of public items.

[qinsoon]

We currently have 368 public items that do not have comments. I plan to add comments for them, and also check if those items really should be public. I probably will submit one PR for every 50 changes or so. The PRs should be removing pub for some items, and adding missing comments.