src: major cleanups, introduce new eXtended Span class

- initial minimally invasive eXtended Span implementation
- rename ptr_diff to ptr_diff_bytes
- move some files to util subdir
- lots of cleanups
- start using the new checked pointers - this needs some real-world testing

src/compress.cpp

@@ -28,7 +28,7 @@
 
 #include "conf.h"
 #include "compress.h"
-#include "mem.h"
+#include "util/membuffer.h"
 
 
 /*************************************************************************