Remove cl and entirely rely on cl-lib instead

[stephan-cr]

Since Emacs 27, cl is deprecated. This PR replaces all functions and macros from cl by corresponding ones from cl-lib.

[stephan-cr]

I've seen the build failures.

$ emacs --version
/tmp/emacs-26.1-travis/bin/emacs: error while loading shared libraries: libgnutls.so.26: cannot open shared object file: No such file or directory
The command "emacs --version" exited with 127.
$ make monky.elc
emacs -Q --batch -f batch-byte-compile monky.el
/tmp/emacs-26.1-travis/bin/emacs: error while loading shared libraries: libgnutls.so.26: cannot open shared object file: No such file or directory
Makefile:8: recipe for target 'monky.elc' failed
make: *** [monky.elc] Error 127
The command "make monky.elc" exited with 2.

and

$ emacs --version
/tmp/emacs-git-snapshot-travis/bin/emacs: error while loading shared libraries: libgnutls.so.26: cannot open shared object file: No such file or directory
The command "emacs --version" exited with 127.
$ make monky.elc
emacs -Q --batch -f batch-byte-compile monky.el
/tmp/emacs-git-snapshot-travis/bin/emacs: error while loading shared libraries: libgnutls.so.26: cannot open shared object file: No such file or directory
Makefile:8: recipe for target 'monky.elc' failed
make: *** [monky.elc] Error 127
The command "make monky.elc" exited with 2.

doesn't relate to my changes.

[Wilfred]

Thanks :)