* [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
Some generic C++ and Rust code is not generated when unions use type
aliases because of potential ambiguity. Actually check for this
ambiguity and only disable offending code only if it is found.
iceboy