AK: Add FlyString, a simple flyweight string class

FlyString is a flyweight string class that wraps a RefPtr<StringImpl>
known to be unique among the set of FlyStrings. The class is very
unoptimized at the moment.

When to use FlyString:

- When you want O(1) string comparison
- When you want to deduplicate a lot of identical strings

When not to use FlyString:

- For strings that don't need either of the above features
- For strings that are likely to be unique

Kernel/Makefile

@@ -1,13 +1,14 @@
 OBJS = \
     ../AK/FileSystemPath.o \
+    ../AK/FlyString.o \
     ../AK/JsonParser.o \
     ../AK/JsonValue.o \
     ../AK/LogStream.o \
     ../AK/String.o \
     ../AK/StringBuilder.o \
     ../AK/StringImpl.o \
-    ../AK/StringView.o \
     ../AK/StringUtils.o \
+    ../AK/StringView.o \
     ../Libraries/LibELF/ELFImage.o \
     ../Libraries/LibELF/ELFLoader.o \
     ../Libraries/LibBareMetal/Output/Console.o \