Split async I/O functionality into a separate 'cppcoro_io' library

[lewissbaker]

As discussed in https://github.com/lewissbaker/cppcoro/issues/46#issuecomment-328516796 the async I/O facilities provided by cppcoro should be split out into a separate library to allow applications to make use of the generic, core components of cppcoro (task, async_generator, when_all etc.) without pulling in the platform-specific async I/O subsystems.

This will allow applications to more easily use other I/O frameworks like libuv, boost::asio or the Networking TS in conjunction with cppcoro.

[IronsDu]

asio already support co_awiat, can both use cppcoro and asio? i think will rewrite it use cppcoro and async asio. maybe persuade asio use cppcoro to support co_await ?

[IronsDu]

Hi, i see asio can't help me translate async to sync(for co_await). so i very hope cppcoro will slit io, and focus generic, core components.