Besides thread-local “keys” (pointer-sized non-addressable thread-local store with an associated destructor), many platforms also provide thread-local destructors that are not associated with any particular data. These are often more efficient.
This module provides a fallback implementation for that interface, based
on the less efficient thread-local “keys”. Each platform provides
thread_local_dtor module which will either re-export the fallback,
or implement something more efficient.