Navigate Cell as Cursor

[oleibman]

Fix #863, which went stale in 2019 and is now reopened. Add methods to Cell which operate in a cursor-like manner, allowing you to change to cells on the right, left, down, or up.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• 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