Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

core/io/stream_peer.cpp

@@ -29,7 +29,8 @@
 /*************************************************************************/
 
 #include "stream_peer.h"
-#include "io/marshalls.h"
+
+#include "core/io/marshalls.h"
 
 Error StreamPeer::_put_data(const PoolVector<uint8_t> &p_data) {