The File struct represents a file that has been opened (it wraps a file
descriptor), and gives read and/or write access to the underlying file.
Since many things can go wrong when doing file I/O, all the File methods
return the IoResult<T> type, which is an alias for Result<T, IoError>.
This makes the failure of all I/O operations explicit, thanks to this the programmer can see all the failure paths, and is encouraged to handle them in a proactive manner.