Define module's hooks with defparameter

When I need to recompile everything, the hooks should be emptied out.

src/modules.lisp

@@ -12,15 +12,15 @@
            :initialize :finalize :update))
 (in-package :stoe.modules)
 
-(defvar *initialize-hook* nil
+(defparameter *initialize-hook* nil
   "Hook run on initialization.
 Functions attached to this hook should expect an optional argument containing
 the program argv.")
 
-(defvar *finalize-hook* nil
+(defparameter *finalize-hook* nil
   "Hook run on finalization.")
 
-(defvar *update-hook* nil
+(defparameter *update-hook* nil
   "Hook run each frame.")
 
 (defun initialize (&optional argv)