#5904: improve support for ppx in the toplevel by allowing ppx processors to keep information across calls (through the use of persistent cookies).  Also change Ast_mapper.register so that the function that creates the mapper from arguments is executed once the context has been restored.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15314 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02