borrowed_reborrowable lint doesn't work with Fn traits or function pointers

[MrGVSV]

This issue was originally meant for closures as per #164 (comment). However, I think I managed to actually get closures working.

What's not being caught by the lint are the Fn traits and function pointers:

• impl FnOnce(&mut Commands)
• Box<dyn FnMut(&mut EntityMut)>
• fn(&mut PtrMut)
• Etc.

This is probably something we can try to handle in the lint as well.