HasGet for structure fields

[notdanilo]

We want to allow this:

#[derive(IsTree)]
#[tree(branches)]
pub struct Structure {
  pub name: String,
  pub surname: String
}

#[test]
fn main() {
  let structure = Structure { name: "Danilo".into(), surname: "Guanabara".into() };
  assert_eq!("{}", structure.get("name").unwrap(), "Danilo");
  assert_eq!("{}", structure.get("surname").unwrap(), "Guanabara");
  assert!(structure.get("invalid").is_none());
}

This is a requirement for a runtime reflection system.