[Go] Make enums into real types, add String() (#5235)

* [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