AK: Introduce cutoff to insertion sort for Quicksort

Implement insertion sort in AK. The cutoff value 7 is a magic number
here, values [5, 15] should work well. Main idea of the cutoff is to
reduce recursion performed by quicksort to speed up sorting
of small partitions.

Tests/AK/CMakeLists.txt

@@ -38,6 +38,7 @@ set(AK_TEST_SOURCES
     TestIPv4Address.cpp
     TestIPv6Address.cpp
     TestIndexSequence.cpp
+    TestInsertionSort.cpp
     TestIntegerMath.cpp
     TestIntrusiveList.cpp
     TestIntrusiveRedBlackTree.cpp