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/util/dt_impl.cpp

@@ -0,0 +1,37 @@
+/* dt_impl.cpp -- doctest support code implementation
+
+   This file is part of the UPX executable compressor.
+
+   Copyright (C) 1996-2022 Markus Franz Xaver Johannes Oberhumer
+   Copyright (C) 1996-2022 Laszlo Molnar
+   All Rights Reserved.
+
+   UPX and the UCL library are free software; you can redistribute them
+   and/or modify them under the terms of the GNU General Public License as
+   published by the Free Software Foundation; either version 2 of
+   the License, or (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; see the file COPYING.
+   If not, write to the Free Software Foundation, Inc.,
+   59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+   Markus F.X.J. Oberhumer              Laszlo Molnar
+   <markus@oberhumer.com>               <ezerotven+github@gmail.com>
+ */
+
+/*************************************************************************
+// doctest support code implementation
+**************************************************************************/
+
+#define DOCTEST_CONFIG_IMPLEMENT
+#define DOCTEST_CONFIG_NO_MULTITHREADING
+#define DOCTEST_CONFIG_NO_UNPREFIXED_OPTIONS
+#include <doctest/parts/doctest.cpp>
+
+/* vim:set ts=4 sw=4 et: */