Refactor library

[adayNU]

Seeks to re-factor the original repo w/ many breaking changes. Major goals:

1. Use more idiomatic go style. (Rename the package, accept and return pointers, shorter variable names, etc.)
2. Much better documentation. Add comments to all exported types, fields, methods, packages, etc. (from OpenAI's documentation).
3. Make option fields who's go default value is a valid parameter value (and also does not match the API's default) pointers, so omitempty does not strip explicitly set values. (See here for more).
4. Have a consistent style throughout. Most endpoints in the original library were implemented independently (and thus their usages feel inconsistent).
5. Implement all endpoints.

Let me know your thoughts (especially around request fields, and whether or not they should be pointers).

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This change is 

[neilmadsen]

Reviewed 34 of 34 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @adayNU)