Implement field_init_shorthand feature

``` Rust
#![feature(field_init_shorthand)]

struct AA{}
struct BB{}

struct CC {
    a: AA,
    b: BB
}

impl CC {
    fn new(a: AA, b: BB) -> Self {
        Self { a, b }
    }
}
```

`cargo fmt` error:
```
error: expected `:`, found `,`
  --> /tmp/foo/src/lib.rs:13:17
   |
13 |         Self { a, b }
   |                 ^

```