Various utils to implement
fumio that are not actually specific to it.
Generic implementation for "current" (thread-local) instances for "executor" handles.
Create a "link" and "head" datatype for a non thread-safe double linked list; the "link" type must be used in a data structure as member.
Create a list "link" and "head" datatype; the "link" type must be used in a data structure as
member; this data structure then can be pushed as