Handling Unions as Function Arguments

[oleibman]

Fix #4656. (Also fix #503, which went stale years ago, and which I reopened, and which I re-closed in a state of confusion.) Continuing the work of PR #4596. Calculation engine was unable to parse a formula which used union arguments. (I find it very difficult to parse as well.) This PR will, I hope, fix that. More tests are needed.

This is:

• [x] a bugfix
• [ ] a new feature
• [ ] refactoring
• [ ] additional unit tests

Checklist:

• [ ] Changes are covered by unit tests
  • [ ] Changes are covered by existing unit tests
  • [x] New unit tests have been added
• [x] Code style is respected
• [x] Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• [ ] CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• [ ] Documentation is updated as necessary

[oleibman]

Back to draft status. Example from https://github.com/PHPOffice/PhpSpreadsheet/discussions/1950 does not work.