Auto merge of #4525 - lukaslueg:springclean, r=alexcrichton

Cleaning lints

I've started to clean some minor defects in cargo. This is the first commit of possibly many.

Requesting advice if this is actually wanted; #cargo was positive.

Some things raise the minimum version of rust required to compile cargo. E.g. `assert_ne!(foo, bar)` instead of `assert!(foo != bar)` requires (iirc) rust 1.13. Any advice on that in particular?