Clippy fixes

[kinnison]

Fix a variety of clippy issues, some by allowing the lints, some by fixing the issues.

I hope this is of use.

[sdroege]

Let's also make clippy warnings fatal in the CI then (and in glib/gio at least, the others need fixing for various clippy warnings first)? :) @GuillaumeGomez ?

[sdroege]

lgtm. @kinnison can you add the magic to make clippy warnings errors to the .travis.yml where it already calls clippy currently?

[kinnison]

@sdroege Yes, but only when travis comes back to tell me I've not fluffed anything else up :D

[GuillaumeGomez]

Let's merge once the clippy warnings are denied then. :)

[kinnison]

I got bored waiting for travis so I pushed what ought to do the trick.

[GuillaumeGomez]

The new failure is because there is already documentation in our repository. I think the best way for the TreeRowReference method is to add it into the repository (sorry about that...).

[kinnison]

@GuillaumeGomez if we move the safety information to the lgpl-docs repo then we also need to ensure we run cargo clippy with the embed-lgpl-docs feature set, I'll try that locally and see if it works

[GuillaumeGomez]

Thanks!