update upf8 from 2.3.4 to 3.2.3

New convenience API for C++ 11 and later compilers. The library still works with C++ 98/03 compliant compilers, just without the new functions.
advance() function works in both directions.
The following deprecated functions were removed:

previous() - deprecated since version 1.02.
is_bom() - deprecated since version 2.3.
Fix of the project version number at CMakeLists.txt
Continuous Integration with Google Tests and CircleCI
A minor release that contains fix for Issue assimp#31 Program fails to link when including utf8.h in multiple files.
This release adds one new API call: unchecked::replace_invalid().
Optional support for C++ 17 std::string_view.
The release contains the fix the inclusion of both cpp11 and cpp17 headers on C++17 compilation. Also some additional tests for using string literals and string objects with modern compilers.

contrib/utf8cpp/doc/LICENSE

@@ -0,0 +1,23 @@
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
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+a source language processor.
+
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+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
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