run clippy on our doc code examples

[oli-obk]

see #1362 for a funny one

[mcarton]

Find a way to integrate clippy into rustdoc?

Most of our example are not complete, so they wouldn't compile anyway.

[oli-obk]

It's not rustdoc, it's libtest. We might be able to inject clippy as a plugin.

[phansch]

This is now doable and partly done with #3845 🎉