Support struct expression shorthand field

[A4-Tacks]

Fields with the same type and name use the 'shorthand' syntax

Example

struct Foo { bar: bool, n: i32 }

fn baz() {
    let bar = true;
    let foo: Foo = Fo$0;
}

Current:

struct Foo { bar: bool, n: i32 }

fn baz() {
    let bar = true;
    let foo: Foo = Foo { bar: ${1:()}, n: ${2:()} }$0;
}

Expect:

struct Foo { bar: bool, n: i32 }

fn baz() {
    let bar = true;
    let foo: Foo = Foo { bar$1, n: ${2:()} }$0;
}

@rustbot label +A-completion