Change `Stream` implementation of `ExpandedSwarm` to return all
`SwarmEvents` instead of only the `NetworkBehaviour`'s events.
Remove `ExpandedSwarm::next_event`. Users can use `<ExpandedSwarm as
StreamExt>::next` instead.
Remove `ExpandedSwarm::next`. Users can use `<ExpandedSwarm as
StreamExt>::filter_map` instead.
This commit extends the ping example in `src/tutorial.rs, by walking a
newcomer through the implementation of a simple ping node step-by-step,
introducing all the core libp2p concepts along the way.
With the ping tutorial in place, there is no need for the lengthy libp2p
crate level introduction, which is thus removed with this commit.
Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>
