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

AK/Forward.h

@@ -45,6 +45,7 @@ class StringBuilder;
 class StringImpl;
 class StringView;
 class URL;
+class FlyString;
 class Utf8View;
 
 template<typename T>
@@ -137,5 +138,6 @@ using AK::StringImpl;
 using AK::StringView;
 using AK::Traits;
 using AK::URL;
+using AK::FlyString;
 using AK::Utf8View;
 using AK::Vector;