[#147] Work around object package

- implement Stringer interface and Parse method for object.ID
- increase test coverage

closes #147

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

pkg/object/id.go

@@ -4,12 +4,19 @@ import (
 	"bytes"
 	"crypto/sha256"
 
+	"github.com/mr-tron/base58"
+	"github.com/nspcc-dev/neofs-api-go/internal"
 	"github.com/nspcc-dev/neofs-api-go/v2/refs"
+	"github.com/pkg/errors"
 )
 
 // ID represents v2-compatible object identifier.
 type ID refs.ObjectID
 
+// ErrBadID should be returned when bytes slice hasn't sha256.Size
+// Notice: if byte slice changed, please, replace error message.
+const ErrBadID = internal.Error("object.ID should be 32 bytes length")
+
 // NewIDFromV2 wraps v2 ObjectID message to ID.
 func NewIDFromV2(idV2 *refs.ObjectID) *ID {
 	return (*ID)(idV2)
@@ -39,3 +46,22 @@ func (id *ID) Equal(id2 *ID) bool {
 func (id *ID) ToV2() *refs.ObjectID {
 	return (*refs.ObjectID)(id)
 }
+
+// Parse converts base58 string representation into ID.
+func (id *ID) Parse(s string) error {
+	data, err := base58.Decode(s)
+	if err != nil {
+		return errors.Wrap(err, "could not parse object.ID from string")
+	} else if len(data) != sha256.Size {
+		return ErrBadID
+	}
+
+	(*refs.ObjectID)(id).SetValue(data)
+
+	return nil
+}
+
+// String returns base58 string representation of ID.
+func (id *ID) String() string {
+	return base58.Encode((*refs.ObjectID)(id).GetValue())
+}