AK: Add a forward declaration header

You can now #include <AK/Forward.h> to get most of the AK types as
forward declarations.

Header dependency explosion is one of the main contributors to compile
times at the moment, so this is a step towards smaller include graphs.

AK/StringBuilder.h

@@ -26,8 +26,8 @@
 
 #pragma once
 
-#include <AK/String.h>
-#include <AK/Vector.h>
+#include <AK/ByteBuffer.h>
+#include <AK/Forward.h>
 #include <stdarg.h>
 
 namespace AK {
@@ -45,8 +45,7 @@ public:
     void appendf(const char*, ...);
     void appendvf(const char*, va_list);
 
-    String build() { return to_string(); }
-
+    String build();
     String to_string();
     ByteBuffer to_byte_buffer();