[#378] accounting: Add binary encoding for Decimal type

Define `Marshal` / `Unmarshal` methods of the `Decimal` type.

Signed-off-by: Leonard Lyubich <ctulhurider@gmail.com>

accounting/decimal.go

@@ -1,6 +1,8 @@
 package accounting
 
-import "github.com/nspcc-dev/neofs-api-go/v2/accounting"
+import (
+	"github.com/nspcc-dev/neofs-api-go/v2/accounting"
+)
 
 // Decimal represents decimal number for accounting operations.
 //
@@ -62,3 +64,30 @@ func (d Decimal) Precision() uint32 {
 func (d *Decimal) SetPrecision(p uint32) {
 	(*accounting.Decimal)(d).SetPrecision(p)
 }
+
+// Marshal encodes Decimal into a binary format of the NeoFS API protocol
+// (Protocol Buffers with direct field order).
+//
+// See also Unmarshal.
+func (d Decimal) Marshal() []byte {
+	var m accounting.Decimal
+	d.WriteToV2(&m)
+
+	return m.StableMarshal(nil)
+}
+
+// Unmarshal decodes NeoFS API protocol binary format into the Decimal
+// (Protocol Buffers with direct field order). Returns an error describing
+// a format violation.
+//
+// See also Marshal.
+func (d *Decimal) Unmarshal(data []byte) error {
+	var m accounting.Decimal
+
+	err := m.Unmarshal(data)
+	if err != nil {
+		return err
+	}
+
+	return d.ReadFromV2(m)
+}

accounting/decimal_test.go

@@ -5,6 +5,7 @@ import (
 
 	v2accounting "github.com/nspcc-dev/neofs-api-go/v2/accounting"
 	"github.com/nspcc-dev/neofs-sdk-go/accounting"
+	accountingtest "github.com/nspcc-dev/neofs-sdk-go/accounting/test"
 	"github.com/stretchr/testify/require"
 )
 
@@ -44,3 +45,12 @@ func TestDecimalMessageV2(t *testing.T) {
 	require.EqualValues(t, d.Value(), m2.GetValue())
 	require.EqualValues(t, d.Precision(), m2.GetPrecision())
 }
+
+func TestDecimal_Marshal(t *testing.T) {
+	d := *accountingtest.Decimal()
+
+	var d2 accounting.Decimal
+	require.NoError(t, d2.Unmarshal(d.Marshal()))
+
+	require.Equal(t, d, d2)
+}