Add the possibility to assume that multistream-select will succeed

[tomaka]

If the connection upgrade only accepts one protocol, we can optimize the multistream-select process by assuming that it works, and closing the connection/substream altogether if later the remote returns "na".

I would suggest modifying dialer_select_proto so that if the size_hint() of the protocols list returns a maximum of 1, immediately succeed after sending the data without waiting for the remote to confirm, and wraps around the returned stream.

The behaviour should also be precisely documented.

[tomaka]

This should be integrated in such a way that multistream-select 2.0 is easy to plug in later.

[tomaka]

I think that multistream-select should simplfy add a Negotiated<T> wrapper around the successfully negotiated substreams (even on the listening side), so that we can do whatever we want underneath.

[tomaka]

Note that shutdown should check the outcome of the negotiation and produce an error if it failed, otherwise failed negotiations would be undetected for write-only substreams.