Automate more checks

[joshka]

The list of checkboxes of things to manually run to check a PR is prohibitively large, which is a barrier to contribution.
Ideally a single run "cargo xtask check" or something similar should be provided that does all the checks.

[AndyGauge]

That would make it super easy to contribute to, which is the direction we want to head.

[Arteiii]

Well, move the current source files to a new dir (cookbook/) and create the xtask and .cargo dir along with a root Cargo.toml for the workspace.

would probably make it easier to run the book build/serve and test?

I have tried this and the current structure looks like this, LMK if this approach is fine for you.

├── .cargo
│   ├── config.toml
├── ci
├── cookbook
│   ├── assets
│   ├── src
│   ├── tests
│   ├── theme
│   ├── book.toml
│   ├── build.rs
│   ├── Cargo.toml
├── xtask
│   ├── src
│   ├── Cargo.toml
│   ├── Cargo.lock
├── appveyor.yml
├── travis.yml
├── Cargo.toml (workspace file)