after all this groundwork, implementing the invocation,
capturing and hand-over of results is simple, and the
thread-wrapper classes became fairly understandable.
This relieves the Thread policy from a lot of technicalities,
while also creating a generally useful tool: the ability to invoke
/anything callable/ (thanks to std::invoke) in a fail-safe way and
transform the exception into an Either type
on second thought, the ability to transport an exception still seems
worthwhile, and can be achieved by some rearrangements in the design.
As preparation, reorganise the design of the Either-wrapper (lib::Result)
