Polish transliterator data struct

[skius]

Tracking a few follow-ups for the data struct:

• Rename compounds into variables
  • Also any references in various documentation, e.g. icu_transliterator_parser
• VarTable => SpecialsTable
• Consistent naming for id_group_list and rule_group_list - both are "rules" according to the spec, "transform rule" vs "conversion rule"
• Go through data struct and check there's no unnecessary MultiFieldsULE being generated
• visibility => visible.
• SimpleId => FilteredTransliterator