[#168] accounting: Implement binary/JSON encoders/decoders on Decimal

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/accounting/decimal.go

@@ -40,3 +40,35 @@ func (d *Decimal) SetPrecision(p uint32) {
 	(*accounting.Decimal)(d).
 		SetPrecision(p)
 }
+
+// Marshal marshals Decimal into a protobuf binary form.
+//
+// Buffer is allocated when the argument is empty.
+// Otherwise, the first buffer is used.
+func (d *Decimal) Marshal(b ...[]byte) ([]byte, error) {
+	var buf []byte
+	if len(b) > 0 {
+		buf = b[0]
+	}
+
+	return (*accounting.Decimal)(d).
+		StableMarshal(buf)
+}
+
+// Unmarshal unmarshals protobuf binary representation of Decimal.
+func (d *Decimal) Unmarshal(data []byte) error {
+	return (*accounting.Decimal)(d).
+		Unmarshal(data)
+}
+
+// MarshalJSON encodes Decimal to protobuf JSON format.
+func (d *Decimal) MarshalJSON() ([]byte, error) {
+	return (*accounting.Decimal)(d).
+		MarshalJSON()
+}
+
+// UnmarshalJSON decodes Decimal from protobuf JSON format.
+func (d *Decimal) UnmarshalJSON(data []byte) error {
+	return (*accounting.Decimal)(d).
+		UnmarshalJSON(data)
+}