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 #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/source/utf8/cpp11.h

@@ -70,7 +70,7 @@ namespace utf8
     inline std::size_t find_invalid(const std::string& s)
     {
         std::string::const_iterator invalid = find_invalid(s.begin(), s.end());
-        return (invalid == s.end()) ? std::string::npos : (invalid - s.begin());
+        return (invalid == s.end()) ? std::string::npos : static_cast<std::size_t>(invalid - s.begin());
     }
 
     inline bool is_valid(const std::string& s)