Create a new Format method that is like StringOfBase but never errors

cid.go

@@ -338,6 +338,18 @@ func (c *Cid) StringOfBase(base mbase.Encoding) (string, error) {
 	}
 }
 
+// Format return the string representation of a Cid
+func (c *Cid) Format(base mbase.Encoder) string {
+	switch c.version {
+	case 0:
+		return c.hash.B58String()
+	case 1:
+		return base.Encode(c.bytesV1())
+	default:
+		panic("not possible to reach this point")
+	}
+}
+
 // Hash returns the multihash contained by a Cid.
 func (c *Cid) Hash() mh.Multihash {
 	return c.hash

cid_test.go

@@ -152,6 +152,15 @@ func TestBasesMarshaling(t *testing.T) {
 		}
 
 		assertEqual(t, cid, out2)
+
+		prefix, err := mbase.NewEncoder(b)
+		if err != nil {
+			t.Fatal(err)
+		}
+		s2 := cid.Format(prefix)
+		if s != s2 {
+			t.Fatalf("'%s' != '%s'", s, s2)
+		}
 	}
 }