Implement `ignoreExistential` attribute.

Records' default selectors are generated using a private `#record` attribute handler. if a record contains existential type, selectors will not be generated at all. This behaviour could be altered using `ignoreExistential` attribute. It would omit fields that require existential type and generate selectors for regular ones.

Placeholder for this attribute is already implemented; currently it does not make any effect on the code, other than providing an additional argument to the `#record` attribute. This feature should be implemented as an extension to the `#record` attribute.