High-level API:
func Transform() cmp.Option
type Enum struct{ ... }
type Message map[string]interface{}
The Transform function transform messages into a Message type that
cmp.Equal and cmp.Diff then knows how to traverse and compare.
Change-Id: I445f3b5c69f054b6984f28c205cda69e44af3b89
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/164680
Reviewed-by: Damien Neil <dneil@google.com>
