add writev_all to io modules

[FrankReh]

And add File::writev_at_all as the first user facing example.

The writev_all function will call the io_uring writev operation as many times as necessary to get all the bytes written or to hit an error, which ever comes first.

[FrankReh]

@ollie-etl , @mzabaluev and others, welcome to review also.

[FrankReh]

@mzabaluev The same function will be called from TcpStream and UnixStream so a common place like the io directory seemed reasonable to me.