Improve integration tests and CI

[jakoschiko]

• Add integration tests to workspace
• Remove Cargo.lock for integration tests, should not be necessary anymore with workspace
• Update CI
  • We don't need to handle integration tests anymore, we can simply use --workspace for most commands.
  • I don't know why you used --all for fmt, I'm not sure if it does what you think it does
  • --check can be used without --
  • I added cargo check
  • I added cargo clippy. I hope it's okay. I can revert this if you want.
• Fix format
• Fix clippy warnings. I can revert this if you want.
• Update edition of integration tests
• Make integration tests not publishable (to prevent accidentally publishing)

[jakoschiko]

I suggest reviewing the commits separately.

[jakoschiko]

Any ideas why this fails on CI? It works locally and I use the same Cargo.lock (because it's committed).

[jakoschiko]

Okay, it seemed to be a problem with MSRV. My changes triggered an update of the Cargo.toml. I was able to fix it locally by running cargo +nightly update -Z minimal-versions. But now the serde test fails. Maybe the test relies on a minor or patch release of serde?

[jakoschiko]

Yep, we need at least serde_json = "1.0.1" :p

I added this to the Cargo.toml