swarm/src/lib: Remove Deref and DerefMut impls on Swarm (#1995)

Remove `Deref` and `DerefMut` implementations previously dereferencing
to the `NetworkBehaviour` on `Swarm`. Instead one can access the
`NetworkBehaviour` via `Swarm::behaviour` and `Swarm::behaviour_mut`.
Methods on `Swarm` can now be accessed directly, e.g. via
`my_swarm.local_peer_id()`.

Reasoning: Accessing the `NetworkBehaviour` of a `Swarm` through `Deref`
and `DerefMut` instead of a method call is an unnecessary complication,
especially for newcomers. In addition, `Swarm` is not a smart-pointer
and should thus not make use of `Deref` and `DerefMut`, see documentation
from the standard library below.

> Deref should only be implemented for smart pointers to avoid
confusion.

https://doc.rust-lang.org/std/ops/trait.Deref.html

protocols/gossipsub/src/lib.rs

@@ -118,7 +118,7 @@
 //!
 //! // Listen on a memory transport.
 //! let memory: Multiaddr = libp2p_core::multiaddr::Protocol::Memory(10).into();
-//! let addr = libp2p_swarm::Swarm::listen_on(&mut swarm, memory).unwrap();
+//! let addr = swarm.listen_on(memory).unwrap();
 //! println!("Listening on {:?}", addr);
 //! ```