fix: incorrect equivalence classes

Summarize the changes

• 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

Ensure that

• New code is covered by unit and integration tests.
• Related issues would be automatically closed with gitlab's closing pattern (Closes #issue_number).
• Public modules are documented (check the rendered version with cargo doc --open).
• (if PEST grammar has changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none
• Update sbroad submodule in picodata/picodata.
• (if EBNF grammar has changed) create a follow-up issue in picodata/docs.