AK: Add CircularBuffer

The class is very similar to `CircularDuplexStream` in its behavior.
Main differences are that `CircularBuffer`:
 - does not inherit from `AK::Stream`
 - uses `ErrorOr` for its API
 - is heap allocated (and OOM-Safe)

 This patch also add some tests.

Tests/AK/CMakeLists.txt

@@ -14,6 +14,7 @@ set(AK_TEST_SOURCES
     TestByteBuffer.cpp
     TestCharacterTypes.cpp
     TestChecked.cpp
+    TestCircularBuffer.cpp
     TestCircularDeque.cpp
     TestCircularDuplexStream.cpp
     TestCircularQueue.cpp