Add support for differently shaped enums variants

[hobofan]

Support specifying per-variant attributes on enum variants that are shaped differently. E.g:

#[derive(Delegate)]
#[delegate(Shout)]
enum {
  Bar(Cat), // will automatically be delegated to first field
  #[delegate(Shout, target = "bar")]
  Foo { bar: Cat, baz: Dog },
}

This will also most likely require more precise error output, or else it will be hard to explain to the user which variant is missing an attribute.