Add hippo.allocate_page()

src/hulk/hippo.d

@@ -102,4 +102,20 @@ struct hippo
         free_pages = pp;
         n_free_pages++;
     }
+
+    /**
+     * Allocate a physical page.
+     *
+     * @return Page address, or null if none available.
+     */
+    public static void * allocate_page()
+    {
+        PhysicalPage * pp;
+        if (pp != null)
+        {
+            pp = free_pages;
+            free_pages = free_pages.next;
+        }
+        return cast(void *)pp;
+    }
 }