Implemented SORT_UNIFY, SORT_UNIQUE and debugged SORT_VAR_DATA.

Implemented SORT_UNIFY, SORT_UNIQUE and debugged SORT_VAR_DATA.

Now the new sorter is equivalent to the old sorter (i.e., the same algorithms),
but with new interface and standing on new foundations. I will write
a simple test suite and then start filling the gaps with something
interesting.