Add missing standard library includes for self-sufficient headers

Fixes builds with newer GCC/libstdc++ that no longer provide <cstdint>,
<cstring>, <cfloat>, <memory>, <algorithm> etc. transitively. Also
disambiguates visit<> calls in taxonomy.h with the full namespace and
casts the character value in IfcCharacterDecoder to uint32_t to silence
ambiguous overload warnings.

src/ifcparse/schema.h

@@ -26,8 +26,10 @@
 #include <algorithm>
 #include <boost/algorithm/string.hpp>
 #include <cctype>
+#include <cstdint>
 #include <iterator>
 #include <map>
+#include <memory>
 #include <string>
 #include <vector>
 #include <optional>