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/LogStream.cpp

@@ -112,4 +112,10 @@ DebugLogStream dbg()
     return stream;
 }
 
+DebugLogStream::~DebugLogStream()
+{
+    char newline = '\n';
+    write(&newline, 1);
+}
+
 }