Deduplicate the trie node encoding and decode code

[tomaka]

Close #45

There are currently two modules that are extremely similar: node_value and proof_node_codec.
This is because a node in a proof is the same encoding as a node in the trie. A proof is simply a subset of the trie.

This PR merges them into a new trie_node.rs module.

trie_node.rs is identical to proof_node_codec.rs except that:

• Decoded now accepts a second template parameter for the type of the slice of the children.
• It has the calculate_merkle_value function of node_value, and node_value::Output is now trie_node::MerkleValueOutput.

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼─────────────────────────────────────────────────────────────────────────────────────
       -2114 ┊ smoldot::trie::node_value::calculate_merkle_value::h1a08e5db1df4bf3c
       -1819 ┊ smoldot::trie::node_value::calculate_merkle_value::hae8592be1c077277
       -1686 ┊ smoldot::trie::node_value::calculate_merkle_value::h7e4624f4826cdd80
       -1639 ┊ smoldot::trie::node_value::calculate_merkle_value::h0120a2bdbda20b0b
       -1639 ┊ smoldot::trie::node_value::calculate_merkle_value::h978c157ab8a4f6c7
       -1560 ┊ smoldot::trie::proof_node_codec::decode::haf5019ff28b96e53
       +1560 ┊ smoldot::trie::trie_node::decode::hdfea7f1c265892fb
       -1058 ┊ smoldot::trie::node_value::calculate_merkle_value::hdd8b7fe7b28a4ccd
        +934 ┊ smoldot::trie::trie_node::calculate_merkle_value::h6e768a76e876b758
        +903 ┊ smoldot::trie::trie_node::calculate_merkle_value::h5d03bd74f43e0db3
        +812 ┊ smoldot::trie::trie_node::encode::he0ccb24812429c95
        +447 ┊ smoldot::trie::trie_node::encode::h9dff752a3890bcc5
        +425 ┊ smoldot::trie::calculate_root::StorageValue::inject::h7947098e496f0d60
        +330 ┊ <&T as core::fmt::Display>::fmt::h5a4a3c6d1b68e313
        -330 ┊ <&T as core::fmt::Display>::fmt::h8dccc91510640d2b
        -327 ┊ <smoldot::trie::proof_node_codec::Error as core::fmt::Debug>::fmt::h3baa60ab5ba68d14
        +327 ┊ <smoldot::trie::trie_node::Error as core::fmt::Debug>::fmt::h90ff66766c13b5f9
        +294 ┊ smoldot::trie::calculate_root::StorageValue::inject::h2d8bc09a9ea7bc1e
        +294 ┊ smoldot::trie::calculate_root::StorageValue::inject::ha643bf8bfc88bd86
        +293 ┊ smoldot::trie::calculate_root::StorageValue::inject::h33c7b8c3ecb5e119
       +2478 ┊ ... and 100 more.
        +635 ┊ Σ [120 Total Rows]