Set a buildable field by consuming a builder

[loganmzz]

If a nested struct also derive Builder, add setter using a FnOnce consuming a nested builder:

#[derive(Builder)]
#[derive(PartialEq,Debug)]
struct Nested {
  string: String,
}

#[derive(Builder)]
#[derive(PartialEq,Debug)]
struct Parent {
  nested: Nested,
}

let built = Parent::builder()
   .build_nested(|nested| nested.string("nested value"))
   .build();

assert_eq!(
  built,
  Parent {
    nested: Nested {
      string: "nested value".into(),
    },
  },
);