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

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)
+		}
 	}
 }