async-graphql 7.2.1

# Union

The definition of a `Union` is similar to an `Interface`, **but with no fields allowed.**.
The implementation is quite similar for `Async-graphql`; from `Async-graphql`'s perspective, `Union` is a subset of `Interface`.

The following example modified the definition of `Interface` a little bit and removed fields.

```rust
# extern crate async_graphql;
use async_graphql::*;

struct Circle {
    radius: f32,
}

#[Object]
impl Circle {
    async fn area(&self) -> f32 {
        std::f32::consts::PI * self.radius * self.radius
    }

    async fn scale(&self, s: f32) -> Shape {
        Circle { radius: self.radius * s }.into()
    }
}

struct Square {
    width: f32,
}

#[Object]
impl Square {
    async fn area(&self) -> f32 {
        self.width * self.width
    }

    async fn scale(&self, s: f32) -> Shape {
        Square { width: self.width * s }.into()
    }
}

#[derive(Union)]
enum Shape {
    Circle(Circle),
    Square(Square),
}
```

## Flattening nested unions

A restriction in GraphQL is the inability to create a union type out of
other union types. All members must be `Object`. To support nested
unions, we can "flatten" members that are unions, bringing their members up
into the parent union. This is done by applying `#[graphql(flatten)]` on each
member we want to flatten.

```rust
# extern crate async_graphql;
#[derive(async_graphql::Union)]
pub enum TopLevelUnion {
    A(A),

    // Will fail to compile unless we flatten the union member
    #[graphql(flatten)]
    B(B),
}

#[derive(async_graphql::SimpleObject)]
pub struct A {
    a: i32,
    // ...
}

#[derive(async_graphql::Union)]
pub enum B {
    C(C),
    D(D),
}

#[derive(async_graphql::SimpleObject)]
pub struct C {
    c: i32,
    // ...
}

#[derive(async_graphql::SimpleObject)]
pub struct D {
    d: i32,
    // ...
}
```

The above example transforms the top-level union into this equivalent:

```rust
# extern crate async_graphql;
# #[derive(async_graphql::SimpleObject)]
# struct A { a: i32 }
# #[derive(async_graphql::SimpleObject)]
# struct C { c: i32 }
# #[derive(async_graphql::SimpleObject)]
# struct D { d: i32 }
#[derive(async_graphql::Union)]
pub enum TopLevelUnion {
    A(A),
    C(C),
    D(D),
}
```