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/ui.h

@@ -25,6 +25,7 @@
    <markus@oberhumer.com>               <ezerotven+github@gmail.com>
  */
 
+#pragma once
 #ifndef __UPX_UI_H
 #define __UPX_UI_H 1