Change attribute syntax to @foo

As with the enum thing, I can commit to pushing the impl through relatively quickly.

[Rendered](https://github.com/sfackler/rfcs/blob/attr-syntax/text/0000-attribute-syntax.md)
Yes, and I'm heartily against the current `#[foo = "bar"]` form as well, which is why I'm so eager to use this opportunity to nix it from the language. :)