Change the way buffers are passed to the wasm-node

[tomaka]

Close #375

This PR modifies the way the JS passes buffers to the Rust.
Instead of calling an alloc function to allocate memory in the Wasm, the JS now locally assigns a "buffer index", and the Rust uses host functions in order to obtain the properties of each "buffer index".

This makes things more simple, and also fixes a potential undefined behavior.

One small wart is that the connection_new function (JS function called by the Rust) requires allocating a buffer index that can only be freed "some time in the future" rather than at a determinate point in time.
However, this is in practice not a problem given that buffer index 0 is constantly written over.

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
        +851 ┊ <futures_util::stream::unfold::Unfold<T,F,Fut> as futures_core::stream::Stream>::poll_next::h697290eadcad6e9c
        -851 ┊ <futures_util::stream::unfold::Unfold<T,F,Fut> as futures_core::stream::Stream>::poll_next::he5c66c7ba77a43a2
        +205 ┊ add_chain
        -132 ┊ futures_util::unfold_state::UnfoldState<T,R>::take_value::h9babf0fa52cb7d6d
        +132 ┊ futures_util::unfold_state::UnfoldState<T,R>::take_value::ha1e5107e3a5bd3ee
        +102 ┊ alloc::raw_vec::RawVec<T,A>::allocate_in::hd536446d9776dbed
        +100 ┊ smoldot_light_wasm::bindings::get_buffer::haeb6592cf46700cc
         +98 ┊ futures_util::unfold_state::_::<impl futures_util::unfold_state::UnfoldState<T,R>>::project_replace::h3e44ab8dea30ccc7
         -98 ┊ futures_util::unfold_state::_::<impl futures_util::unfold_state::UnfoldState<T,R>>::project_replace::he55c536e1f12eada
         -94 ┊ alloc
         -93 ┊ core::result::Result<T,E>::unwrap::h96ab1208f15e76e6
         +80 ┊ data[0]
         -79 ┊ core::array::<impl core::ops::index::Index<I> for [T; N]>::index::h553e97ecd944e4cb
         +73 ┊ futures_util::unfold_state::UnfoldState<T,R>::project_future::h17016c0b103031fe
         -73 ┊ futures_util::unfold_state::UnfoldState<T,R>::project_future::h3c8f854c79006575
         -71 ┊ <smoldot_light_wasm::platform::Platform as smoldot_light::platform::Platform>::connect::h13c215900d2e995e
         +67 ┊ stream_message
         +63 ┊ connection_reset
         +59 ┊ core::ptr::drop_in_place<futures_util::unfold_state::UnfoldState<smoldot_light::JsonRpcResponses,smoldot_light_wasm::add_chain::{{closure}}::{{closure}}::{{closure}}>>::h1172919e8188cfbf
         -59 ┊ core::ptr::drop_in_place<futures_util::unfold_state::UnfoldState<smoldot_light::JsonRpcResponses,smoldot_light_wasm::add_chain::{{closure}}::{{closure}}::{{closure}}>>::h5721b0538a646ba3
         +39 ┊ ... and 41 more.
        +282 ┊ Σ [61 Total Rows]