Run clippy on code examples

[qwandor]

Running it on the code in Cargo workspaces should be straightforward. It is unclear how to do so on inline code samples.

[mgeisler]

It is unclear how to do so on inline code samples.

Yeah, for that we would need to extend mdbook test — or perhaps this could be something we could extend mdbook-keeper (see #175) to do? I've talked with the mdbook authors before about extending and improving mdbook test, and I get the impression that they would prefer to offload the testing functionality.