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/variant_array.h

@@ -33,6 +33,10 @@ variant - which is the maximum size of its constituents - is reduced.
 #include <utility>
 #include <memory>
 #include <tuple>
+#include <cstdint>
+#include <cstring>
+#include <cstddef>
+#include <limits>
 
 #include "exception.h"