Consider to import all tag enumeration values into the choice class

[mikir]

Example of the current solution for choices:

zserioChoice.emplace<ConditionValue::Tag::CHOICE_dateRangeOfYear>(computeDateRangeOfYear(value));

Idea is to import all choice tag enumeration values directly into the choice class to simplify the previous example to:

zserioChoice.emplace<ConditionValue::CHOICE_dateRangeOfYear>(computeDateRangeOfYear(value));

However, this will introduce the clashes with the choice keywords (e.g. Base) and with the variant base class keywords (e.g. index). These clashes must be carefully solved. It is not acceptable just to forbid field names like index in the schema.

Reported by @reinco.