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/StringView.h

@@ -55,6 +55,7 @@ public:
 
     StringView(const ByteBuffer&);
     StringView(const String&);
+    StringView(const FlyString&);
 
     bool is_null() const { return !m_characters; }
     bool is_empty() const { return m_length == 0; }