Decode Merkle proofs entirely at once

[tomaka]

cc #2973

This PR rewrites the code that verifies Merkle proofs. Instead of iterating through the proof while decoding it to find one specific element, we now fully decode and verify the proof then later find the elements from that decoding. This means that we no longer need to verify the proof multiple times.

In terms of API, this means that decoding the proof no longer returns any "entry missing" error. The errors about a missing entry have thus been added at a higher level.

The logic of the code keeps track of all the entries found in the proof. Later, in order to find an element, we actually iterate "upwards". So for example to find 0xf00bab, we first try 0xf00bab, then for example 0xf00b and then 0xf00. The vast majority of the time there should be a precise hit, so it seems better to me to try to find 0xf00bab first and then iterate upward in order to prove that the requested key doesn't exist or isn't in the proof.

I've removed the existing code. Despite the fact that it works, I think it's a strictly worse way of doing it than the code in this PR.

As I've reached the end of this PR and my brain is fried, I realized that using a BTreeMap is kind of stupid since the proof is already a tree, and we should instead just keep a parallel Vec adding some information that avoids having to recalculate hashes when decoding the tree. However I don't have the courage to do this anymore and I've just left a TODO instead. Since this is purely an implementation detail and that the complexity would remain the same, it can be changed later without breaking anything.

I also had in mind to immediately add a next_key and a prefix_key function to the proof, but again I don't have the courage to do so now.

[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.

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 Delta Bytes │ Item
─────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────
       +6668 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h67e45a83a0270f06
       -6552 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h9321c12a0a9e32e8
       +2223 ┊ smoldot::sync::warp_sync::BuildChainInformation<TSrc,TRq>::build::he71aeeb4a8e19549
       +2138 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h8e4e65654e5d6353
       +2133 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h9f8e3f0deb4d58d6
       +2096 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hb9e63801c83cb8e9
       +2094 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hafb856c3e168f443
       +2071 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hc8460297ebb56e76
       +2039 ┊ smoldot::trie::proof_decode::decode_and_verify_proof::h2296938b316dcd09
       +2039 ┊ smoldot::trie::proof_decode::decode_and_verify_proof::h833adbae05bf9768
       +2039 ┊ smoldot::trie::proof_decode::decode_and_verify_proof::h88a8812ccc0f70a7
       +2018 ┊ smoldot::trie::proof_decode::decode_and_verify_proof::h191e66faca1841cb
       +2010 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hcedc31f861e0a76f
       +2010 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hf56f483ce62c33c4
       -1979 ┊ smoldot::sync::warp_sync::BuildRuntime<TSrc,TRq>::build::he9de39c758fcbdbb
       +1979 ┊ smoldot::sync::warp_sync::BuildRuntime<TSrc,TRq>::build::hfd10369ab6509fc7
       +1978 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hbff43cf1ea49b729
       +1864 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h7b776881750092c4
       +1844 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::he60f4b97c8fcc335
       -1823 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h07035515b1b45a7a
       -2884 ┊ ... and 407 more.
      +41531 ┊ Σ [427 Total Rows]