Language FAQ is out of date

[catamorphism]

https://github.com/mozilla/rust/wiki/Doc-language-FAQ

The code example is clearly out of date. Didn't look if the rest of the FAQ needs updating too.

[catamorphism]

I updated the code example and the obviously out-of-date things in the questions and answers. I didn't try compiling the code example, so I probably made a mistake. For now, though, I'm happier with the FAQ.