Remove default features. You need to enable required features
explicitly now. As a quick workaround, you may want to use the
new `full` feature which activates all features.
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`.
Previously `libp2p-swarm` required a `Transport` to be `Clone`. Methods
on `Transport`, e.g. `Transport::dial` would take ownership, requiring
e.g. a `Clone::clone` before calling `Transport::dial`.
The requirement of `Transport` to be `Clone` is no longer needed in
`libp2p-swarm`. E.g. concurrent dialing can be done without a clone per
dial.
This commit removes the requirement of `Clone` for `Transport` in
`libp2p-swarm`. As a follow-up methods on `Transport` no longer take
ownership, but instead a mutable reference (`&mut self`).
On the one hand this simplifies `libp2p-swarm`, on the other it
simplifies implementations of `Transport`.
