fix: incorrect equivalence classes

- `propagate_equality` transformation did not
compute equality classes correctly, its 'merge'
function was completely wrong: it tried to add
intersection of classes to a another class,
instead of doing union
- to merge classes correctly we must do it
when we add a new pair of equal expressions:
otherwise later there will too many classes
that contain common elements, so 'merge'
function was removed and 'insert' now merges
two classes that contain common elements
- Also this logic is now covered by tests