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/StringImpl.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/Badge.h>
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 #include <AK/Types.h>
@@ -70,11 +71,15 @@ public:
         return m_hash;
     }
 
+    bool is_fly() const { return m_fly; }
+    void set_fly(Badge<FlyString>, bool fly) const { m_fly = fly; }
+
 private:
     enum ConstructTheEmptyStringImplTag {
         ConstructTheEmptyStringImpl
     };
     explicit StringImpl(ConstructTheEmptyStringImplTag)
+        : m_fly(true)
     {
         m_inline_buffer[0] = '\0';
     }
@@ -89,6 +94,7 @@ private:
     size_t m_length { 0 };
     mutable unsigned m_hash { 0 };
     mutable bool m_has_hash { false };
+    mutable bool m_fly { false };
     char m_inline_buffer[0];
 };