Move detail::bind_handler into asio namespace

[justend29]

bind_handler binds the given arguments to the provided handler while preserving the handler's associations through partial specializations of associator.

When creating asynchronous operations, the above function is cleaner and is more space efficient than chaining calls to `bind_(executor | allocator | immediate_executor | cancellation_slot ), as numerous copies of the associations aren't stored in the resulting object. This is what asio does for internal async operations.

bind_handler should be moved into the public asio namespace such that it's available on the reference page and users become aware of it.