rlp: optimize byte array handling (#22924)

This change improves the performance of encoding/decoding [N]byte.

    name                     old time/op    new time/op    delta
    DecodeByteArrayStruct-8     336ns ± 0%     246ns ± 0%  -26.98%  (p=0.000 n=9+10)
    EncodeByteArrayStruct-8     225ns ± 1%     148ns ± 1%  -34.12%  (p=0.000 n=10+10)

    name                     old alloc/op   new alloc/op   delta
    DecodeByteArrayStruct-8      120B ± 0%       48B ± 0%  -60.00%  (p=0.000 n=10+10)
    EncodeByteArrayStruct-8     0.00B          0.00B          ~     (all equal)

rlp/encode_test.go

@@ -513,3 +513,22 @@ func BenchmarkEncodeConcurrentInterface(b *testing.B) {
 	}
 	wg.Wait()
 }
+
+type byteArrayStruct struct {
+	A [20]byte
+	B [32]byte
+	C [32]byte
+}
+
+func BenchmarkEncodeByteArrayStruct(b *testing.B) {
+	var out bytes.Buffer
+	var value byteArrayStruct
+
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		out.Reset()
+		if err := Encode(&out, &value); err != nil {
+			b.Fatal(err)
+		}
+	}
+}