turn `am` into a full workspace

[mellowagain]

the cargo workspace now has three members:

• am: the actual cli binary
• autometrics-am: the accompanying library of am
• am_list: self explanatory

im kinda not that happy with the naming right now, maybe we should rename the library to something which makes more sense than autometrics-am? i can imagine contributors might get confused about what it is compared to am. im open for ideas, i thought about smth like libam

fixes #129

Checklist

• [x] Changelog updated

[gagbo]

It should be fine with things. That mostly reminds me that the GitHub action needs to be updated.