state: optimize NEP17Balance serialization

Put to slice directly and allow to provide pre-allocated buffer.
```
BenchmarkNEP17BalanceBytes/stackitem-8         	 1712475	       673.4 ns/op	     448 B/op	       9 allocs/op
BenchmarkNEP17BalanceBytes/bytes-8             	13422715	        75.80 ns/op	      32 B/op	       2 allocs/op
BenchmarkNEP17BalanceBytes/bytes,_prealloc-8   	25990371	        46.46 ns/op	      16 B/op	       1 allocs/op
```

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/core/native/native_gas.go

@@ -59,7 +59,7 @@ func (g *GAS) increaseBalance(_ *interop.Context, _ util.Uint160, si *state.Stor
 	}
 	acc.Balance.Add(&acc.Balance, amount)
 	if acc.Balance.Sign() != 0 {
-		*si = acc.Bytes()
+		*si = acc.Bytes(nil)
 	} else {
 		*si = nil
 	}

pkg/core/state/native_state.go

@@ -7,6 +7,7 @@ import (
 	"math/big"
 
 	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neo-go/pkg/encoding/bigint"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 )
 
@@ -33,8 +34,20 @@ func NEP17BalanceFromBytes(b []byte) (*NEP17Balance, error) {
 }
 
 // Bytes returns serialized NEP17Balance.
-func (s *NEP17Balance) Bytes() []byte {
+func (s *NEP17Balance) Bytes(buf []byte) []byte {
-	return balanceToBytes(s)
+	if cap(buf) < 4+bigint.MaxBytesLen {
+		buf = make([]byte, 4, 4+bigint.MaxBytesLen)
+	} else {
+		buf = buf[:4]
+	}
+	buf[0] = byte(stackitem.StructT)
+	buf[1] = 1
+	buf[2] = byte(stackitem.IntegerT)
+
+	data := bigint.ToPreallocatedBytes(&s.Balance, buf[4:])
+	buf[3] = byte(len(data)) // max is 33, so we are ok here
+	buf = append(buf, data...)
+	return buf
 }
 
 func balanceFromBytes(b []byte, item stackitem.Convertible) error {

pkg/core/state/native_state_test.go

@@ -0,0 +1,50 @@
+package state
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNEP17Balance_Bytes(t *testing.T) {
+	var b NEP17Balance
+	b.Balance.SetInt64(0x12345678910)
+
+	data, err := stackitem.SerializeConvertible(&b)
+	require.NoError(t, err)
+	require.Equal(t, data, b.Bytes(nil))
+
+	t.Run("reuse buffer", func(t *testing.T) {
+		buf := make([]byte, 100)
+		ret := b.Bytes(buf[:0])
+		require.Equal(t, ret, buf[:len(ret)])
+	})
+}
+
+func BenchmarkNEP17BalanceBytes(b *testing.B) {
+	var bl NEP17Balance
+	bl.Balance.SetInt64(0x12345678910)
+
+	b.Run("stackitem", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			_, _ = stackitem.SerializeConvertible(&bl)
+		}
+	})
+	b.Run("bytes", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			_ = bl.Bytes(nil)
+		}
+	})
+	b.Run("bytes, prealloc", func(b *testing.B) {
+		bs := bl.Bytes(nil)
+
+		b.ResetTimer()
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			_ = bl.Bytes(bs[:0])
+		}
+	})
+}