items/union example triggers float literal compatibility lint

[kyrias]

The unions section has an example matching on a float literal, which triggers the rust-lang/rust#41620 compatibility lint:

#[repr(u32)]
enum Tag { I, F }

#[repr(C)]
union U {
 i: i32,
 f: f32,
}

#[repr(C)]
struct Value {
 tag: Tag,
 u: U,
}

fn is_zero(v: Value) -> bool {
 unsafe {
 match v {
 Value { tag: I, u: U { i: 0 } } => true,
 Value { tag: F, u: U { f: 0.0 } } => true,
 _ => false,
 }
 }
}