Improve return type for Collection::partition (#380)

Currently when partitioning a Collection-object it will return an array
of ReadableCollections unless that return-type is overwritten in the
implementation.

This change makes sure that any implementation of a Collection actually
returns an array of Collections instead.

src/Collection.php

@@ -100,4 +100,14 @@ public function map(Closure $func);
      * @psalm-return Collection<TKey, T>
      */
     public function filter(Closure $p);
+
+    /**
+     * {@inheritDoc}
+
+     * @return Collection<mixed>[] An array with two elements. The first element contains the collection
+     *                      of elements where the predicate returned TRUE, the second element
+     *                      contains the collection of elements where the predicate returned FALSE.
+     * @psalm-return array{0: Collection<TKey, T>, 1: Collection<TKey, T>}
+     */
+    public function partition(Closure $p);
 }