[rust/en] Add an English Rust tutorial

Fixes #480.
We (should) support everything for which there is a lexer in my pointlessly-forked version of Rouge (which only exists because middleman-syntax demands a 3.x version instead of the current 4.x series, even though the api is the same).

Which is to say, if there's a rust lexer in https://github.com/adambard/rouge/tree/master/lib/rouge/lexers, we support it. And, it looks like there is! How unexpected.