* is_completion_handler is deprecated
* type_traits.hpp is a deprecated include
These items will be removed in the next version
* Include the new header file for the types needed:
- stream_traits.hpp
- file_base.hpp
* Use std::is_invocable instead of is_completion_handler
* Tidying
* Increase test coverage
* Fix file_mode::append_existing
API Changes:
* file_mode::append_new is removed, as it makes no sense
Actions Required:
* Replace file_mode::append_new with file_mode::append
or file_mode::append_existing instead of file_mode::append_new
