Adds lint for Vec<Box<T: Sized>>
This adds, and subsequently closes#3530. This is the first time I've ever worked with anything remotely close to internal Rust code, so I'm very much unsure about the if_chain! to figure this out!
I can't get rustfmt working on WSL with nightly 2018-12-07:
`error: component 'rustfmt' for target 'x86_64-unknown-linux-gnu' is unavailable for download`