ENH Add generic types

There are also a few general corrections to PHPDocs that I noticed along
the way (e.g. adding `|null` when the method is returning a null value.

There are some cases where either the return type or the whole PHPDoc
was duplicated from the parent class - in those cases I've simply
removed the duplication.

src/ORM/ArrayList.php

@@ -29,6 +29,12 @@
  *   - sort
  *   - filter
  *   - exclude
+ *
+ * @template T
+ * @implements SS_List<T>
+ * @implements Filterable<T>
+ * @implements Sortable<T>
+ * @implements Limitable<T>
  */
 class ArrayList extends ViewableData implements SS_List, Filterable, Sortable, Limitable
 {
@@ -116,6 +122,8 @@ public function exists()
     /**
      * Returns an Iterator for this ArrayList.
      * This function allows you to use ArrayList in foreach loops
+     *
+     * @return Traversable<T>
      */
     public function getIterator(): Traversable
     {
@@ -130,8 +138,6 @@ public function getIterator(): Traversable
 
     /**
      * Return an array of the actual items that this ArrayList contains.
-     *
-     * @return array
      */
     public function toArray()
     {
@@ -342,11 +348,6 @@ public function shift()
         return array_shift($this->items);
     }
 
-    /**
-     * Returns the first item in the list
-     *
-     * @return mixed
-     */
     public function first()
     {
         if (empty($this->items)) {
@@ -356,11 +357,6 @@ public function first()
         return reset($this->items);
     }
 
-    /**
-     * Returns the last item in the list
-     *
-     * @return mixed
-     */
     public function last()
     {
         if (empty($this->items)) {