Support embedding enums

[ptrus]

Would be great if the following use-case of embedding enums would be supported:

#[derive(Clone, Debug, PartialEq, cbor::Encode, cbor::Decode)]
pub enum B { 
  #[cbor(rename="b_variant")]
  BVariant,
  #[cbor(rename="b_variant2")]
  BVariant2,
}

#[derive(Clone, Debug, PartialEq, cbor::Encode, cbor::Decode)]
pub enum A {
  #[cbor(rename="a_variant")]
  AVariant,
  #[cbor(embed)]
  Embed(B),
}

Where A would result in a "merged/flattened" enum being able to deserialize serialied B enums.

Current workaround requires manually "embedding" B enum vairants into A:

#[derive(Clone, Debug, PartialEq, cbor::Encode, cbor::Decode)]
pub enum A {
  #[cbor(rename="a_variant")]
  AVariant,
  #[cbor(rename="b_variant")]
  BVariant,
  #[cbor(rename="b_variant2")]
  BVariant2,
}

Related serde issue:

• #[serde(flatten)] attribute support for enum variants serde-rs/serde#1402