Implement encoding proof node values

[tomaka]

Necessary in order to finish #1166

Right now we can decode items from Merkle proofs. This PR implements encoding back these items.

While implementing encoding, I've discovered several cases where the decoding was too lenient and was accepting as valid things that shouldn't really be valid. I've fixed this as part of this PR. See the changes in src/util.rs and at the bottom of src/trie/proof_node_codec.rs.

When I say "shouldn't really be valid" I mean that there were multiple different sequences of bytes that could lead to the same decoded struct. Since we're hashing things a lot here and there, it is important that there is a bijection of decoded <-> encoded. The Substrate implementation does the same. cc paritytech/substrate#12417

[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
─────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
       +1453 ┊ smoldot::trie::proof_node_codec::decode::h8c2a3387e8fd00db
       -1398 ┊ smoldot::trie::proof_node_decode::decode::h3c802fc74e097b79
        +309 ┊ <&T as core::fmt::Display>::fmt::hc61d54e835a5e55a
        +306 ┊ <smoldot::trie::proof_node_codec::Error as core::fmt::Debug>::fmt::h771dc1f7d38ceb18
        -267 ┊ <&T as core::fmt::Display>::fmt::hede33647e967f354
        -264 ┊ <smoldot::trie::proof_node_decode::Error as core::fmt::Debug>::fmt::h52238c6df391e999
        +257 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h24d08d1fb54a1500
        -257 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h505ccd594b2d1511
        -176 ┊ alloc::vec::Vec<T,A>::extend_desugared::h2508a4c0bc10f95c
        +176 ┊ alloc::vec::Vec<T,A>::extend_desugared::hdafe000385f163e9
        -119 ┊ <smoldot::trie::nibble::nibbles_to_bytes_extend::Iter<I> as core::iter::traits::iterator::Iterator>::next::hcceeebdd458373a3
        +119 ┊ <smoldot::trie::nibble::nibbles_to_bytes_suffix_extend::Iter<I> as core::iter::traits::iterator::Iterator>::next::hfb9289ced9322d6a
        +111 ┊ <smoldot::trie::proof_node_codec::PartialKey as core::iter::traits::iterator::Iterator>::next::hf090f35ddcd91518
        -111 ┊ <smoldot::trie::proof_node_decode::PartialKey as core::iter::traits::iterator::Iterator>::next::h7efb59290db7c70f
        +107 ┊ smoldot::trie::proof_node_codec::Decoded::children_bitmap::h47da02d2a0a90f55
        -107 ┊ smoldot::trie::proof_node_decode::Decoded::children_bitmap::hebf4eaa59ed5d772
         +98 ┊ smoldot::util::nom_scale_compact_usize::h78e936afad2da3eb
         +98 ┊ smoldot::util::nom_scale_compact_usize::hf6e357ea1a4d52cf
         +83 ┊ <smoldot::trie::proof_node_codec::PartialKey as core::clone::Clone>::clone::he6bae938fa7cc5af
         -83 ┊ <smoldot::trie::proof_node_decode::PartialKey as core::clone::Clone>::clone::h2e2a851a709e4791
         +97 ┊ ... and 14 more.
        +453 ┊ Σ [34 Total Rows]

[melekes]

👍