c++20 preparation workarounds

[JonnyH]

This allows us to build with c++20 enabled - the biggest changes are the introduction of char8_t for utf8 character streams, and char32_t and similar no longer being implicitly cast-able to other integer types (As they're meant to be utf32 codepoints, not "numbers")

Note this is a /workaround/ - a long-term solution would be to replace the entire UString type with u8string (as they effectively mean the same thing - "utf8-encoded bytes" - and move the helpers we use to free functions (if they can't be replaced by standard library functions).

Unfortunately, pre-c++20 didn't define all the types (e.g. char8_t being distinct from 'char'), so until we move completely to only even newer c++ versions we probably need some level of hackery.

[Atrosha]

When I set c++20 compiler options I get an errors on building with clang 10.0.0
gcc works fine.

/home/atrosha/OpenApoc/game/state/tilemap/collision.cpp:49:19: error: use of overloaded operator '!=' is ambiguous (with operand types 'OpenApoc::LineSegmentIterator<int, true>' and 'OpenApoc::LineSegmentIterator<int, true>')
        for (auto &point : line)
                         ^
/home/atrosha/OpenApoc/library/line.h:131:7: note: candidate function
        bool operator!=(const LineSegmentIterator &other) { return !(*this == other); }
             ^
/home/atrosha/OpenApoc/library/line.h:126:7: note: candidate function
        bool operator==(const LineSegmentIterator &other)
             ^
/home/atrosha/OpenApoc/library/line.h:126:7: note: candidate function (with reversed parameter order)
/home/atrosha/OpenApoc/game/state/tilemap/collision.cpp:49:21: note: in implicit call to 'operator!=' for iterator of type 'OpenApoc::LineSegment<int, true>'
        for (auto &point : line)
                           ^~~~
/home/atrosha/OpenApoc/library/line.h:27:39: note: selected 'begin' function with iterator type 'LineSegmentIterator<int, true>'
        LineSegmentIterator<T, conservative> begin() const;
                                             ^
1 warning and 1 error generated.
make[2]: *** [game/state/CMakeFiles/OpenApoc_GameState.dir/build.make:970: game/state/CMakeFiles/OpenApoc_GameState.dir/tilemap/collision.cpp.o] Error 1