use cl- prefixed symbols from cl-lib.el

instead of the prefix-less variants from cl.el

Starting with version 24.3 Emacs comes with library cl-lib which defines cl- prefixed versions of the CL functions that we previously were only allowed to use at compile time. For users of older Emacsen a forward port is available from the GNU Elpa repository.

I created this patch against next because we might want to wait for Emacs-24.3 to be released before applying this to master. However the patch also applies to master without any merge conflicts.