Add an action to convert structure update syntax to explicit field passing

i.e. consider this:

```rust
fn f() {
    struct S { a: u8, b: u8, c: u8 }

    let s = S { a: 0, b: 1, c: 2 };
    let s = S { b: 4, ..s };
    //                ^^^ cursor somewhere here
}
```
==>>
```rust
fn f() {
    struct S { a: u8, b: u8, c: u8 }

    let s = S { a: 0, b: 1, c: 2 };
    let s = S { b: 4, a: s.a, c: s.c };
}
```