Move invoke, alloc, and continuation helpers out of detail

[vinnie.falco@…]

Currently these files are in boost/asio/detail/:

handler_alloc_helpers.hpp
handler_cont_helpers.hpp
handler_invoke_helpers.hpp

I believe that these should not be a detail but rather, part of the official asio interface. The rationale is that if someone wants to write a handler wrapper that provides the same io_service execution guarantees (for example, something like boost::detail::rewrapped_handler), they cannot do so efficiently without relying on detail functions.

[chris_kohlhoff]

Why do you think these are necessary? If possible, please provide a use case and an example. I have implemented the hooks in other projects without using anything out of the asio::detail namespace. Mostly this stuff just exists to support old compilers.