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/decode_test.go

@@ -26,6 +26,8 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+
+	"github.com/ethereum/go-ethereum/common/math"
 )
 
 func TestStreamKind(t *testing.T) {
@@ -1063,7 +1065,7 @@ func ExampleStream() {
 	// [102 111 111 98 97 114] <nil>
 }
 
-func BenchmarkDecode(b *testing.B) {
+func BenchmarkDecodeUints(b *testing.B) {
 	enc := encodeTestSlice(90000)
 	b.SetBytes(int64(len(enc)))
 	b.ReportAllocs()
@@ -1078,7 +1080,7 @@ func BenchmarkDecode(b *testing.B) {
 	}
 }
 
-func BenchmarkDecodeIntSliceReuse(b *testing.B) {
+func BenchmarkDecodeUintsReused(b *testing.B) {
 	enc := encodeTestSlice(100000)
 	b.SetBytes(int64(len(enc)))
 	b.ReportAllocs()
@@ -1093,6 +1095,44 @@ func BenchmarkDecodeIntSliceReuse(b *testing.B) {
 	}
 }
 
+func BenchmarkDecodeByteArrayStruct(b *testing.B) {
+	enc, err := EncodeToBytes(&byteArrayStruct{})
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.SetBytes(int64(len(enc)))
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	var out byteArrayStruct
+	for i := 0; i < b.N; i++ {
+		if err := DecodeBytes(enc, &out); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkDecodeBigInts(b *testing.B) {
+	ints := make([]*big.Int, 200)
+	for i := range ints {
+		ints[i] = math.BigPow(2, int64(i))
+	}
+	enc, err := EncodeToBytes(ints)
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.SetBytes(int64(len(enc)))
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	var out []*big.Int
+	for i := 0; i < b.N; i++ {
+		if err := DecodeBytes(enc, &out); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
 func encodeTestSlice(n uint) []byte {
 	s := make([]uint, n)
 	for i := uint(0); i < n; i++ {