type of NAME slot of thread-related datastructures
The name of THREAD can be an arbitrary object.
The name of MUTEX, WAITQUEUE, SEMAPHORE must be
a simple-string if given.
The name of a SB-QUEUE:QUEUE can be an arbitrary object
We should canonicalize this.
I'd suggest (OR NULL SYMBOL SIMPLE-STRING).
What would be an appropriate name for DEFTYPE'ing this?