Create a new trait CrateDefType to retrieve definiton type

[celinval]

The goal would be to provide a trait to allow users to access tcx.type_of() for definitions that have a type, such as FnDef, InstanceDef, StaticDef...

Something like:

trait CrateDefType : CrateDef {
  fn ty(&self) -> Ty {
    with(|cx| cx.type_of(self.def_id()))
  }
}

I think this should be a separate trait, since not every CrateDef has a type. Thus, only the ones that do should implement the new trait.