allow event-based actors to become detached

this patch extends the `spawn` function to allow `spawn<Impl,blocking_api>(...)`
and thereby allow users to detach event-based actors using a class;
this patch also removes some unneeded headers, renames
`detail::recursive_queue_node` to `mailbox_element`, combines the two classes
`ge_reference_wrapper` and `ge_mutable_reference_wrapper` to simply
`rebindable_reference`, and does some maintenance and documentation