swarm-derive/: Don't fail when ignored fields are first

[mxinden]

The NetworkBehaviour derive macro seems to fail when the #[behaviour(ignore)] fields are not the very last fields.

E.g. while the first here fails, the second succeeds:

/// Behaviour type.
#[derive(libp2p::NetworkBehaviour)]
#[behaviour(out_event = "BehaviourEvent", event_process = true)]
pub struct Behaviour<Types: IpfsTypes> {
    #[behaviour(ignore)]
    repo: Arc<Repo<Types>>,
    // mdns: Toggle<TokioMdns>,
    kademlia: Kademlia<MemoryStore>,
    #[behaviour(ignore)]
    kad_subscriptions: SubscriptionRegistry<KadResult, String>,
    bitswap: Bitswap,
    ping: Ping,
    identify: Identify,
    pubsub: Pubsub,
    pub swarm: SwarmApi,
}

/// Behaviour type.
#[derive(libp2p::NetworkBehaviour)]
#[behaviour(event_process = true)]
pub struct Behaviour<Types: IpfsTypes> {
    // mdns: Toggle<TokioMdns>,
    kademlia: Kademlia<MemoryStore>,
    bitswap: Bitswap,
    ping: Ping,
    identify: Identify,
    pubsub: Pubsub,
    pub swarm: SwarmApi,
    #[behaviour(ignore)]
    repo: Arc<Repo<Types>>,
    #[behaviour(ignore)]
    kad_subscriptions: SubscriptionRegistry<KadResult, String>,
}

See rs-ipfs/rust-ipfs#499 (comment) for a failure in the wild.

[rand0m-cloud]

Specifically, the errors generated from the bad example are the following:

error[E0599]: no method named `into_inner` found for struct `KademliaHandler` in the current scope
  --> src/p2p/behaviour.rs:25:10
   |
25 | #[derive(libp2p::NetworkBehaviour)]
   |          ^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `KademliaHandler<QueryId>`
   |
   = note: this error originates in the derive macro `libp2p::NetworkBehaviour` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `into_inner` found for struct `KademliaHandlerProto` in the current scope
  --> src/p2p/behaviour.rs:25:10
   |
25 | #[derive(libp2p::NetworkBehaviour)]
   |          ^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `KademliaHandlerProto<QueryId>`
   |
   = note: this error originates in the derive macro `libp2p::NetworkBehaviour` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0599`.