SequenceRule refactoring.

1. Rename SequenceRule->IllegalSequenceRule for consistency with other
   rules that have negative semantics.
2. Rename SequenceData->IllegalSequenceData for consistency.
3. Add IllegalSequence type to describe sequences (e.g. differing case,
   variant forms).
4. Create EnIllegalSequenceData enum with common sequences (alpha, numeric,
   keyboard).
Drop the 'En' prefix. I don't think it's necessary since we are only going to provide 'En' characters by default.