The concept type traits are renamed for consistency,
and consolidated into a single header file <beast/core/type_traits.hpp>
A new section, Core Concepts, is added to the documentation describing all
of the core utility classes and functions. This also includes a complete
explanation and sample program describing how to write asynchronous initiation
functions and their associated composed operations.
* Add handler_ptr test and increase coverage
* Add test for prepare_buffer
* Move is_call_possible tests to a .cpp file
* Tidy up docs and declarations
