Optimize pinned page sorting

Previously we would count the pinned objects on each comparison. Since
sorting is O(N log N) and we calculated this on both left and right
pages on each comparison this resulted in a extra iterations over the
slots.

gc.c

@@ -844,6 +844,7 @@ enum {
 struct heap_page {
     short total_slots;
     short free_slots;
+    short pinned_slots;
     short final_slots;
     struct {
 	unsigned int before_sweep : 1;
@@ -7731,9 +7732,13 @@ count_pinned(struct heap_page *page)
 static int
 compare_pinned(const void *left, const void *right, void *dummy)
 {
-    int left_count = count_pinned(*(struct heap_page * const *)left);
-    int right_count = count_pinned(*(struct heap_page * const *)right);
-    return right_count - left_count;
+    struct heap_page *left_page;
+    struct heap_page *right_page;
+
+    left_page = *(struct heap_page * const *)left;
+    right_page = *(struct heap_page * const *)right;
+
+    return right_page->pinned_slots - left_page->pinned_slots;
 }
 
 static int
@@ -7760,6 +7765,7 @@ allocate_page_list(rb_objspace_t *objspace, page_compare_func_t *comparator)
 
     list_for_each(&heap_eden->pages, page, page_node) {
         page_list[i++] = page;
+        page->pinned_slots = count_pinned(page);
         GC_ASSERT(page != NULL);
     }
     GC_ASSERT(total_pages > 0);