move headers from src to src/avian

This is necessary to avoid name conflicts on various platforms.  For
example, iOS has its own util.h, and Windows has a process.h.  By
including our version as e.g. "avian/util.h", we avoid confusion with
the system version.

src/codegen/targets.cpp

@@ -10,7 +10,7 @@
 
 #include <avian/vm/codegen/targets.h>
 
-#include "environment.h"
+#include "avian/environment.h"
 
 namespace avian {
 namespace codegen {