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.cpp

@@ -28,6 +28,7 @@
 #include <AK/Memory.h>
 #include <AK/String.h>
 #include <AK/StringView.h>
+#include <AK/FlyString.h>
 #include <AK/Vector.h>
 
 namespace AK {
@@ -39,6 +40,13 @@ StringView::StringView(const String& string)
 {
 }
 
+StringView::StringView(const FlyString& string)
+    : m_impl(string.impl())
+    , m_characters(string.characters())
+    , m_length(string.length())
+{
+}
+
 StringView::StringView(const ByteBuffer& buffer)
     : m_characters((const char*)buffer.data())
     , m_length(buffer.size())