Auto merge of #5018 - brotzeit:clippy, r=alexcrichton

apply clippy suggestions

I want to try if this actually works =)

There are many other clippy suggestions. I wonder if I can take them for granted or if some of them would be refused.

For example:
```
warning: Constants have by default a `'static` lifetime
  --> src/cargo/lib.rs:53:23
   |
53 | pub const CARGO_ENV: &'static str = "CARGO";
   |                      -^^^^^^^---- help: consider removing `'static`: `&str`
   |
   = note: #[warn(const_static_lifetime)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.185/index.html#const_static_lifetime
```