core/muxing: Flatten StreamMuxer interface to poll_{inbound,outbound,address_change,close} (#2724)

Instead of having a mix of `poll_event`, `poll_outbound` and `poll_close`, we
flatten the entire interface of `StreamMuxer` into 4 individual functions:

- `poll_inbound`
- `poll_outbound`
- `poll_address_change`
- `poll_close`

This design is closer to the design of other async traits like `AsyncRead` and
`AsyncWrite`. It also allows us to delete the `StreamMuxerEvent`.

protocols/relay/CHANGELOG.md

@@ -4,6 +4,8 @@
 
 - Expose `HOP_PROTOCOL_NAME` and `STOP_PROTOCOL_NAME`. See [PR 2734].
 
+- Update to `libp2p-core` `v0.35.0`.
+
 [PR 2734]: https://github.com/libp2p/rust-libp2p/pull/2734/
 
 # 0.10.0

protocols/relay/Cargo.toml

@@ -17,7 +17,7 @@ either = "1.6.0"
 futures = "0.3.1"
 futures-timer = "3"
 instant = "0.1.11"
-libp2p-core = { version = "0.34.0", path = "../../core", default-features = false }
+libp2p-core = { version = "0.35.0", path = "../../core", default-features = false }
 libp2p-swarm = { version = "0.38.0", path = "../../swarm" }
 log = "0.4"
 pin-project = "1"