Improve documentation

[tekktrik]

This one was a doozy! I feel like there were a lot of places where I used bytes and circuitpython_typing.ReadableBuffer would have been fine, but the intention was really to use bytes so I did that.

Also added some documentation updates related to type annotation, but this library would benefit from more thorough documentation of parameters.

Resolves #19

UPDATE

This PR further improves documentation by cleaning up a lot of the module, class, and method docstrings, then adding the missing modules to api.rst so said documentation will be displayed.

Also, this library seemed to have an undocumented dependency on pyasn1 so I added it to setup.py and requirements.txt.

[tekktrik]

Scope of the PR has increased, so updated initial comment.

[tekktrik]

Going to separate out the changes from #25 from this PR, draft for now

[tekktrik]

Ew, ugly git commit history! Will resubmit.

[kattni]

@tekktrik We don't mind lengthy git histories. You don't need to close PRs and resubmit them solely for that reason. Please feel free to stick with PRs regardless of the git history. Thanks for all you've been doing!