Clarify platform reading API

[tomaka]

At the moment, the read_buffer function of the Platform trait (representing how the light client binds to the environment) returns either Some in normal situations or None if the connection has been either closed or reset.

This PR turns this Option into an enum with three variants: Open, Closed, and Reset.
This lets the code properly differentiate between these two latter situations.

Additionally, I've renamed "closed" to "reset" in the entire Wasm<->JS code (all the functions and variables), for clarity. While the Platform trait lets you report that the reading side is closed but the writing side still open, the JS code can't represent that state. Instead, it's either "connection open" or "connection reset".

This PR fixes #2782. The issue was that we were treating read_buffer() returning None as meaning "reading side closed", but actually the entire connection was already reset. We would then call send on the already-reset connection, leading to the error. This kind of confusion is exactly why I've performed the rename.

[mergify]

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

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
       -8588 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h0711ca0ba338a70d
       +8588 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h384a1c5c7acfe64d
       -3556 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hdd0fe18794daa58f
       +3553 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hd88c036a7cac2580
       +2612 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h5d3970607b95455c
       -2577 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hf2dded58c85b01ef
        +787 ┊ alloc::collections::btree::remove::<impl alloc::collections::btree::node::Handle<alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Mut,K,V,alloc::collections::btree::node::marker::Leaf>,alloc::collections::btree::node::marker::KV>>::remove_leaf_kv::h16fa840a475d6707
        -787 ┊ alloc::collections::btree::remove::<impl alloc::collections::btree::node::Handle<alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Mut,K,V,alloc::collections::btree::node::marker::Leaf>,alloc::collections::btree::node::marker::KV>>::remove_leaf_kv::h5be42361ece6d245
        +768 ┊ alloc::collections::btree::node::BalancingContext<K,V>::bulk_steal_right::h03acf36ea99d4452
        -768 ┊ alloc::collections::btree::node::BalancingContext<K,V>::bulk_steal_right::h783d7952d52251d7
        -727 ┊ alloc::collections::btree::node::BalancingContext<K,V>::bulk_steal_left::h25cb0910f5571522
        +727 ┊ alloc::collections::btree::node::BalancingContext<K,V>::bulk_steal_left::hcbc5e5470c24cb9b
        -679 ┊ alloc::collections::btree::fix::<impl alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Mut,K,V,alloc::collections::btree::node::marker::LeafOrInternal>>::fix_node_through_parent::h4f241eb21db42704
        +679 ┊ alloc::collections::btree::fix::<impl alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Mut,K,V,alloc::collections::btree::node::marker::LeafOrInternal>>::fix_node_through_parent::h902389da6c188376
        +603 ┊ alloc::collections::btree::node::Handle<alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Mut,K,V,alloc::collections::btree::node::marker::Leaf>,alloc::collections::btree::node::marker::Edge>::insert_recursing::hb25621d28e7c54fc
        -603 ┊ alloc::collections::btree::node::Handle<alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Mut,K,V,alloc::collections::btree::node::marker::Leaf>,alloc::collections::btree::node::marker::Edge>::insert_recursing::hc58f3055b6604b73
        -585 ┊ alloc::collections::btree::node::BalancingContext<K,V>::do_merge::h48bbf604b993b3a7
        +585 ┊ alloc::collections::btree::node::BalancingContext<K,V>::do_merge::h89a88adfe44feb1b
        -585 ┊ alloc::collections::btree::node::BalancingContext<K,V>::do_merge::h906c794f25d9ab0a
        +585 ┊ alloc::collections::btree::node::BalancingContext<K,V>::do_merge::h963d820094dd7cbc
        +318 ┊ ... and 188 more.
        +587 ┊ Σ [208 Total Rows]