ZMACRO_Stringify was being used in only one place, and it wasn't doing the two-level macro thing, so I don't think it was actually as generally useful as one might think.
Rename ZDList.h-->Dlist.h.
ZThread::sCreate throws when it can't create a thread, so we can reliably return true/false for failure to start the thread, as well as for absence of Callable.
Rename ZMemory.*-->Memory.*