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/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])
+		}
+	})
+}