remove manifest implementation of unsafe_{of,to}_string

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15040 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
remove some uses of unsafe_of_string

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15041 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02