Modularize mechanism reservation tracking

[rcahoon]

Description

Refactor the code that tracks and asserts which Command has ownership of each Subsystem, so that it's more modular.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Be detailed so that your code reviewer can understand exactly how much and what kinds of testing were done, and which might still be worthwhile to do.

• Unit tests: [Add your description here]
• Simulator testing: [Add your description here]
• On-robot bench testing: [Add your description here]
• On-robot field testing: [Add your description here]

[dejabot]

nice! lg overall - mainly have some questions about intended usage and structure of the ReservingCommand wrapper and static methods.