AK: Reduce header dependency graph of String.h

String.h no longer pulls in StringView.h. We do this by moving a bunch
of String functions out-of-line.

AK/FlyString.cpp

@@ -28,6 +28,7 @@
 #include <AK/HashTable.h>
 #include <AK/String.h>
 #include <AK/StringUtils.h>
+#include <AK/StringView.h>
 
 namespace AK {
 
@@ -98,4 +99,9 @@ FlyString FlyString::to_lowercase() const
     return String(*m_impl).to_lowercase();
 }
 
+StringView FlyString::view() const
+{
+    return { characters(), length() };
+}
+
 }