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).
> I would call this class 'Sequence'

Fair enough

> I don't find the term original or variant to be meaningful and I don't think providing these as Strings versus char[][] provides any more clarity.

Agree on the naming feedback, but the use of Strings goes much farther than simply character specification. The use of the `IllegalSequence` class clarified if not simplified the implementation of `IllegalSequenceRule`.