Warn on conflicting names

The choice of names of the visit_* method could probably warn in case of conflicting names for different types. See for example this post: https://discuss.ocaml.org/t/visitors-ppx-with-types-from-functor-input-modules/1637

The crux of the issue is that in

type t = Leaf | Node of {left:t; value:Elt.t; right:t}
[@@deriving visitors {variety="iter"}]

both t and Elt.t get the method visit_t which results in the less-than-friendly error message:

Error: This expression has type Elt.t but an expression was expected of type t