ΛEnumTypes map exceeds golang limits on large yang models

[steiler]

When working with very large yang models like for instance the Nokia SROS once (https://github.com/nokia/7x50_YangModels/tree/master/latest_sros_22.2), on code generation, the resulting ΛEnumTypes map exceeds the golang limits for decleratevely defined maps as described in https://github.com/golang/go/issues/33437#issuecomment-517855365.