Add support for attribute on struct literal field

[joshlf]

rustfmt is currently unable to format code which uses the new (and unstable) attributes on struct fields feature. Demonstration:

$ echo '
> struct Foo {
>     foo: usize,
>     #[cfg(feature = "include-bar")]
>     bar: usize,
> }
> 
> fn new_foo() -> Foo {
>     Foo { 
>         foo: 0,
>         #[cfg(feature = "include-bar")]
>         bar: 0,
>     }
> }
> ' | rustfmt
error: expected identifier, found `#`
  --> stdin:14:9
   |
14 |         #[cfg(feature = "include-bar")]
   |         ^

Interestingly, the same code on the Rust playground is simply formatted to remove the #[cfg(feature = "include-bar")] annotation, producing:

struct Foo {
    foo: usize,
    #[cfg(feature = "include-bar")]
    bar: usize,
}

fn new_foo() -> Foo {
    Foo { foo: 0, bar: 0 }
}

[nrc]

I think this just needs us to update the parser (Syntex)