Add setf method for alexandria:ensure-gethash

I often want to do things like have hash tables that are list-valued (i.e., default to nil) and that can be set with push without having to worry about whether they have already been set (as an aside, Python's defaultdict provides this same facility in a different way).

So I added the following setf method for ensure-gethash:

(#+sbcl sb-ext:without-package-locks
 #-sbcl progn
 (defsetf alexandria:ensure-gethash (key table &optional default) (value)
   (let ((tempvar (gensym "TMP")))
     `(let ((,tempvar (alexandria:ensure-gethash ,key ,table ,default)))
        (declare (ignorable ,tempvar))
        (setf (gethash ,key ,table) ,value)))))