Update comment in contravariant test

The previous definition was actually describing covariance.
Fixing to describe contravariance while keeping 'static in the definition was tricky so just changed to use 'short and 'long.
auto merge of #12150 : jakerr/rust/patch-1, r=pnkfelix

The current comment actually describes *co*-variance.
Fixing this to describe contravariance while keeping 'static in the definition was tricky so just changed to use 'short and 'long.

I found the typo in my attempt to understand the concept of variance itself and the comment confused me. I mention this to point out that I'm new to the concept so may have still got the definition wrong, so please review with care :)
Make std::util private and reexport in prelude.
Remove unnecessary use declarations.
Move replace and swap to std::mem. Get rid of std::util