Rename Pcty_fun to Pcty_arrow (and idem in Types, Typedtree, Outcometree) to be coherent with Ptyp_arrow.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13536 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02