Split from #209

This lets any non-`#[turbo_tasks::value]` struct be debug-formatted properly when inside another `#[turbo_tasks::value]` struct.

e.g.

```rust
#[turbo_tasks::value]
struct Value1 {
  value2: Value2,
}


#[derive(TraceRawVcs, Serialize, Debug, Deserialize, PartialEq, Eq)
struct Value2 {
  value3: Value3Vc
}

#[turbo_tasks::value]
struct Value3 {
  str: StringVc
}
```

Prints:

```rust
Value1 {
    value2: Value2 {
        value3: Value3Vc {
            node: TaskCell(
                TaskId {
                    id: 58,
                },
                0,
            ),
        },
    },
}
```

Replacing the `Debug` derive with `ValueDebugFormat` for `Value2` prints:

```rust
Value1 {
    value2: Value2 {
        value3: Value3 {
          str: "hello!"
        },
    },
}
```