Fixing make/with-collector so that this function should be more compl…

…ient

Collector functions (as by make-collector) no longer return their arguments
which makes their item list dynamic-extent.

From the spec:
The value of a rest parameter is permitted, but not required, to share
structure with the last argument to apply.

We saw no actual bugs with this, but it seemed prudent to fix it before
we did

collectors.lisp

@@ -92,21 +92,18 @@ A Collector function will collect, into a list, all the values
 passed to it in the order in which they were passed. If the
 callector function is called without arguments it returns the
 current list of values."
-  (let ((value initial-value)
-        (cdr (last initial-value)))
+  ;; by using this head cons cell we can simplify the loop body
+  (let* ((head (cons :head (copy-list initial-value)))
+         (cdr (last head)))
     (lambda (&rest items)
-      (unless collect-nil (setf items (delete-if #'null items)))
-      (if items
-          (progn
-            (if value
-                (if cdr
-                    (setf (cdr cdr) items
-                          cdr (last items))
-                    (setf cdr (last items)))
-                (setf value items
-                      cdr (last items)))
-            items)
-          value))))
+      (declare (dynamic-extent items))
+      (if (null items)
+          (cdr head)
+          (loop for i in items
+                when (or collect-nil i)
+                  do (let ((new-cons (cons i nil)))
+                       (setf (cdr cdr) new-cons
+                             cdr new-cons)))))))
 
 (defun make-appender (&optional initial-value)
   "Create an appender function.