[feature request] Callback on before/after setter

[tower120]

It would be nice to have some kind of callback for GENERAL setter, like:

struct S{
  #[getset(get, set_w_callback(update))]
  f1: f32,
  #[getset(get, set_w_callback(update))]
  f2: f32,
  s: f32,
}

impl S{
   fn update(&mut self){
      self.s = f1+f2;
   }
}

Function, that would be called before/after default setter work.