Positive coverage: prove_place_is_movable / field

3 tests exercised this rule:

Source location: tests/borrowck.rs:270

fn foo() -> Datum {
    let x: Pair = Pair { first: Datum { value: 1_u32 }, second: Datum { value: 2_u32 } };
    let a: Datum = x.first;
    let b: Datum = x.second;
    return b;
}

Source location: tests/codegen.rs:141

fn main() -> () {
    let p: Pair = Pair { x: 10_i32, y: 20_i32 };
    println!(p.x);
    println!(p.y);
}

Source location: tests/codegen.rs:264

fn main() -> () {
    let w: Wrapper<i32> = Wrapper::<i32> { val: 42_i32 };
    println!(w.val);
}