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.cpp

@@ -27,7 +27,7 @@
 #include <AK/PrintfImplementation.h>
 #include <AK/StdLibExtras.h>
 #include <AK/StringBuilder.h>
-#include <stdarg.h>
+#include <AK/String.h>
 
 namespace AK {
 
@@ -96,6 +96,11 @@ String StringBuilder::to_string()
     return String((const char*)m_buffer.data(), m_length);
 }
 
+String StringBuilder::build()
+{
+    return to_string();
+}
+
 StringView StringBuilder::string_view() const
 {
     return StringView { (const char*)m_buffer.data(), m_length };