File I/O

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 io::Result<T> type, which is an alias for Result<T, io::Error>.

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.