Previously all of the into_raw_fd implementations only returns a copy of
the inner RawFd, while still holding the ownership of the file
descriptor when returning for into_raw_fd. Since `self` is dropped at
the end of into_raw_fd, the returned file descriptor will actually be
closed, render the function unuseable.
The patch makes sure that into_raw_fd actually takes the ownership of
the file descriptor all the way from the inner IoHandle. To achieve
this, I have to use an Option in IoHandle to store the I/O source. It's
not pretty, but I cannot come up with a better way.
* Add future::poll_fn
* Replace all uses of poll_fn with the new one
* Remove some uses of futures
* Simplify ReadDir and DirEntry
* Remove some use of futures from File
* Use futures subcrates
* Fix imports in docs
* Remove futures-util dependency
* Remove futures-executor-preview
* Refactor
* Require more features in the futures-preview crate
