Implement encoding proof node values (#2819)

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

bin/fuzz/Cargo.toml

@@ -86,8 +86,8 @@ test = false
 doc = false
 
 [[bin]]
-name = "proof-node-decode"
-path = "fuzz_targets/proof-node-decode.rs"
+name = "proof-node-codec"
+path = "fuzz_targets/proof-node-codec.rs"
 test = false
 doc = false
 

bin/fuzz/fuzz_targets/proof-node-decode.rs → bin/fuzz/fuzz_targets/proof-node-codec.rs

@@ -18,5 +18,18 @@
 #![no_main]
 
 libfuzzer_sys::fuzz_target!(|data: &[u8]| {
-    let _ = smoldot::trie::proof_node_decode::decode(data);
+    if let Ok(decoded) = smoldot::trie::proof_node_codec::decode(data) {
+        assert_eq!(
+            smoldot::trie::proof_node_codec::encode(decoded.clone()).fold(
+                Vec::new(),
+                |mut a, b| {
+                    a.extend_from_slice(b.as_ref());
+                    a
+                }
+            ),
+            data,
+            "{:?}",
+            decoded
+        );
+    }
 });

bin/light-base/src/runtime_service.rs

@@ -892,7 +892,9 @@ impl<'a> RuntimeCallLock<'a> {
                     | proof_verify::StorageValue::HashKnownValueMissing(_)
             ) {
                 assert_eq!(key.len() % 2, 0);
-                output.push(trie::nibbles_to_bytes_extend(key.iter().copied()).collect::<Vec<_>>());
+                output.push(
+                    trie::nibbles_to_bytes_suffix_extend(key.iter().copied()).collect::<Vec<_>>(),
+                );
             }
 
             match node_info.children {