perf(trie): prealloc in Nibble::unpack (#5629)

crates/primitives/benches/nibbles.rs

@@ -0,0 +1,19 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use reth_primitives::trie::Nibbles;
+
+/// Benchmarks the nibble unpacking.
+pub fn nibbles_benchmark(c: &mut Criterion) {
+    c.bench_function("Nibbles unpack", |b| {
+        let raw = (1..=32).collect::<Vec<u8>>();
+        b.iter(|| {
+            Nibbles::unpack(&raw);
+        })
+    });
+}
+
+criterion_group! {
+    name = benches;
+    config = Criterion::default();
+    targets = nibbles_benchmark
+}
+criterion_main!(benches);