Be more careful with renaming in Subst
Be more careful with renaming in Subst

If idents occur in two places (e.g. in an expansion), make sure
they always end up with the same Identifier

Signed-off-by: Jon Ludlam <44f878afe53efc66b76772bd845eb65944ed8232@recoil.org>

ocaml-doc/odoc,ocaml-doc/odoc