This commit removes the `Network` abstraction, thus managing `Listeners`
and the connection `Pool` in `Swarm` directly. This is done under the
assumption that noone uses the `Network` abstraction directly, but
instead everyone always uses it through `Swarm`. Both `Listeners` and
`Pool` are moved from `libp2p-core` into `libp2p-swarm`. Given that they
are no longer exposed via `Network`, they can be treated as an
implementation detail of `libp2p-swarm` and `Swarm`.

This change does not include any behavioural changes.

This change has the followin benefits:

- Removal of `NetworkEvent`, which was mostly an isomorphism of
  `SwarmEvent`.
- Removal of the never-directly-used `Network` abstraction.
- Removal of now obsolete verbose `Peer` (`core/src/network/peer.rs`)
  construct.
- Removal of `libp2p-core` `DialOpts`, which is a direct mapping of
  `libp2p-swarm` `DialOpts`.
- Allowing breaking changes to the connection handling and `Swarm` API
  interface without a breaking change in `libp2p-core` and thus a
  without a breaking change in `/transport` protocols.

This change enables the following potential future changes:

- Removal of `NodeHandler` and `ConnectionHandler`. Thus allowing to
  rename `ProtocolsHandler` into `ConnectionHandler`.
- Moving `NetworkBehaviour` and `ProtocolsHandler` into `libp2p-core`,
  having `libp2p-xxx` protocol crates only depend on `libp2p-core` and
  thus allowing general breaking changes to `Swarm` without breaking all
  `libp2p-xxx` crates.

…iour