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.

AK/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(AK_SOURCES
     Assertions.cpp
     Base64.cpp
+    CircularBuffer.cpp
     DeprecatedString.cpp
     FloatingPointStringConversions.cpp
     FlyString.cpp