Add fuutre::then(ex, fct).
update doc for future/shared_future::then(Executor, Fct).
The added commented lines should work, but don't.
Added shared_ptr.hpp.
Trying to use std::function for executors::work. This doesn't works yet.
Make invoker copyable and movable following Boost.Move.
Make nullary_function copyable and movable following Boost.Move. Make use of std::shared_ptr when available.
Added shared_future::then executor.
Clarified issue #6782 which can now be closed.
added missing file.
Merge branch 'develop' of github.com:boostorg/thread into develop
Try to address #9569.