Use UTF-8 exclusively internally

This removes SystemString, SystemChar, etc.
All filepaths and log strings are assumed to be UTF-8,
with conversions to UTF-16 for Windows APIs as appropriate.

Updates amuse, athena, boo, kabufua and nod

DataSpec/DNAMP2/CMDL.cpp

@@ -5,7 +5,7 @@ namespace DataSpec::DNAMP2 {
 
 bool CMDL::Extract(const SpecBase& dataSpec, PAKEntryReadStream& rs, const hecl::ProjectPath& outPath,
                    PAKRouter<PAKBridge>& pakRouter, const DNAMP2::PAK::Entry& entry, bool, hecl::blender::Token& btok,
-                   std::function<void(const hecl::SystemChar*)>) {
+                   std::function<void(const char*)>) {
   /* Check for RigPair */
   CINF cinf;
   CSKR cskr;