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.
Implement List's any() function

This is needed for cases where we only need to know if a list item
matches the given predicate (eg. in Servo, we need to know if attributes
from different DOM elements are equal).
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 :)
auto merge of #12132 : brunoabinader/rust/list-matches-predicate, r=alexcrichton

This is needed for cases where we only need to know if a list item matches the given predicate (eg. in Servo, we need to know if attributes from different DOM elements are equal).
Change `xfail` directives in compiletests to `ignore`, closes #11363