Add two caches to the peers state machine

[tomaka]

The Peers state machine has two functions: unfulfilled_desired_outbound_substreams and fulfilled_undesired_outbound_substreams. They return a list of peer-substreams tuples.

These two functions are called very frequently (every time something happens on the networking), they are O(n) (they iterate over every single substream of every single peer), and most of the time they return nothing.

This PR adds two new fields to the peers state machine with the same names as these functions.
The functions now simply return the content of the field, and the field is updated pro-actively whenever something else changes.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[tomaka]

Thanks to the debug_asserts!, I'm quite confident that the logic is correct. Every single time we make sure that the value is in there if it should be in there, or not in there if it shouldn't be in there. All the debug_asserts are run pretty frequently.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────
       +5146 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::next_event::h980ff9df389960cc
       -4192 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::next_event::h2c2d02ee4e653fb3
       +1140 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::set_peer_notifications_out_desired::hab52a248e2bbb5a5
       +1007 ┊ hashbrown::raw::RawTable<T,A>::reserve_rehash::h658b7f14370a3b29
       +1007 ┊ hashbrown::raw::RawTable<T,A>::reserve_rehash::hbeebf15ee4222fae
        -704 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::set_peer_notifications_out_desired::he5f941a9ffa40cd8
        +646 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::new::hf06eaab3702f6e13
        -606 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::new::h85fd8ef6ea9d3abc
        +557 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::close_out_notification::h74a44eb6af6bdfd3
        -495 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::try_clean_up_peer::h18aedf208639263d
        +495 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::try_clean_up_peer::hc88e6d9d994a4b15
        +347 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::open_out_notification::h12a81db841987995
        -341 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::close_out_notification::hbe99fc51ea335164
        -315 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::open_out_notification::h043a4466a921454d
        -309 ┊ core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut::hafc10ee55589d1f7
        -294 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::broadcast_notification::h5f1e15478bd0008a
        +294 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::broadcast_notification::he22b11517363a9af
        +287 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::connection_id_for_peer::hce194e99c000138e
        -287 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::connection_id_for_peer::hdedb363bcc7251af
        -265 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::add_multi_stream_outgoing_connection::h27635b17c9f943eb
       +3259 ┊ ... and 154 more.
      +10607 ┊ Σ [174 Total Rows]

[melekes]

👍