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/main.cpp

@@ -1122,7 +1122,7 @@ int upx_main(int argc, char *argv[]) {
     argv0 = argv[0];
 
     upx_compiler_sanity_check();
-    if (!upx_doctest_check()) {
+    if (!upx_doctest_check(argc, argv)) {
         fprintf(stderr, "%s: internal error: doctest failed\n", argv0);
         e_exit(EXIT_INIT);
     }