Clean up GCId and GCIdMark

GCIdMarkAndRestore is not really needed, we can let the normal GCIdMark have the restore semantics and get rid of GCIdMarkAndRestore. GCId and GCIdMark can also be cleaned up a bit, for example. the GCIdMark::_gc_id member is unnecessary, some functions return "const uint" instead of "uint", etc.