Switch to a record-based API for Ast_mapper, so that it can be used within the compiler itself (where objects are banned).  The class-based version will be shipped as part of the ppx_tools package.  Only the ifdef example has been adapted in experimental/frisch.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14178 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02