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.

AK/CMakeLists.txt

@@ -5,6 +5,7 @@ set(AK_SOURCES
     DeprecatedFlyString.cpp
     DeprecatedString.cpp
     FloatingPointStringConversions.cpp
+    FlyString.cpp
     Format.cpp
     FuzzyMatch.cpp
     GenericLexer.cpp