Refactor gateway crate

[zeylahellyer]

Refactor the internals of the gateway crate to be easier to read and to increase our bread truck scenario. The goal is to refactor our internals; the API should stay the same as a result of this refactoring, but this may surface changes that may be preferable to make outside of this work. When this is done, mostly anyone should be able to read our code. This also means documenting the code and the private API. :)

[7596ff]

Since the entire crate is refactored by #1809, I'll add it to that issue and close the component ones.

[vilgotf]

Done in next, Twilight 0.14