Completely use cl-lib and lexical-binding

Previously both cl-lib.el and cl.el were used, now use only cl-lib.el.
Use lexical-binding where needed instead of requiring cl just for
`lexical-let`.
Replace some add-to-list with cl-pushnew as add-to-list is not
recommended in lisp program and anyway doesn't work properly with
lexical binding.

mu4e/mu4e-icalendar.el

@@ -41,8 +41,7 @@
 
 (require 'mu4e)
 (require 'gnus-icalendar)
-
-(eval-when-compile (require 'cl))
+(require 'cl-lib)
 
 ;;;###autoload
 (defun mu4e-icalendar-setup ()