Stable Toolchain

[alexytsu]

• move integration tests in the src tree
• migrate frc46_test_actor to new build
• migrate integration test actors to the new build script

TODO in followup: #218

[coveralls]

coverage: 87.128% (+0.4%) from 86.776% when pulling 45848ed on alex/stable-toolchain into bf90078 on main.

[Stebalien]

So, the downside of specifying stable is that CI will break every time rust releases a new version and adds additional warnings. The upside is that you instantly get notified of new warnings/issues, it can just be a little annoying when you're trying to merge a PR and suddenly CI decides to break for unrelated reasons).