* [Go] Make enums into real types, add String()
This changes the generated code for enums: instead of type aliases,
they're now distinct types, allowing for better type-checking. Some
client code may have to be changed to add casts.
Enum types now have a String() method, so they implement fmt.Stringer.
An EnumValues map is now generated, in addition to the existing
EnumNames map, to easily map strings to values.
Generated enum files are now gofmt-clean.
Fixes#5207
* use example.ColorGreen explicitly
* use valid enum value in mutation test, add new test for "invalid" enum
* add length check and comment
718ddea558