Testing, examples, docs, CI

[killercup]

While we have compile-tests for the guides and examples, we currently don't really have a good testing story. We have doc tests for the functions we provide, but nothing in regard of integration tests. We should change that.

My idea is this to add full crates to examples/ instead of in-crate binaries.

Each example:

• has a Cargo.toml, src/main.rs
• depends on the current quicli version as well as other necessary crates
• has a tests/integration.rs that uses a crate like assert_cli to actually test that the correct things happen

I propose we do it like this:

• Convert existing example(s) to this structure (Examples and tests #64)
• Write integration tests for these examples (Add integration test for verbosity example #65)
• Create a Cargo workspace and add examples to prevent compiling libs over and over again (Examples and tests #64)
• Convert waltz call in doc/_test.sh to write to examples/<guide-name> (Examples and tests #64)
• Link to full code in guides
• Add link checker to CI (e.g. using this)
• Fail CI when running waltz creates a diff (Examples and tests #64)
• Write integration tests for code in the guides