AK: Implement FlyString for the new String class

This implements a FlyString that will de-duplicate String instances. The
FlyString will store the raw encoded data of the String instance: If the
String is a short string, FlyString holds the String::ShortString bytes;
otherwise FlyString holds a pointer to the Detail::StringData.

FlyString itself does not know about String's storage or how to refcount
its Detail::StringData. It defers to String to implement these details.

Tests/AK/CMakeLists.txt

@@ -30,6 +30,7 @@ set(AK_TEST_SOURCES
     TestFixedPoint.cpp
     TestFloatingPoint.cpp
     TestFloatingPointParsing.cpp
+    TestFlyString.cpp
     TestFormat.cpp
     TestGenericLexer.cpp
     TestHashFunctions.cpp