Implement *inlined* refined syntax for structs

[nilehmann]

Now we can write

flux_rs::flux! {

struct Range refined by [a: int, b: int]
{
    start: i32[a],
    end: {i32[b] | b >= a},
}

fn test(r: Range[@a, @b]) -> i32{v: v >= 0} {
    r.end - r.start
}

}