Fix cl-lib related warnings

Fixed a few simple issues related to the recent switch to using `cl-lib`
from `cl`, which caused the package to fail to
load (https://github.com/nex3/perspective-el/issues/33).

- Add `cl-` prefix to `defun`, `loop`, and `return-from` expressions.

- Add back `(require 'cl)` because `lexical-let` is not defined in
  `cl-lib`.  Note that `lexical-let` has been deprecated since emacs
  24.1 (https://www.gnu.org/software/emacs/manual/html_node/cl/
  Obsolete-Lexical-Binding.html), true emacs binding should be used
  instead.

Ref #33