* add alternate file
inspired by vim ctrl-6/kak ga commands. the alternate file is kept per view
* apply feedback from #223
* rename to last_accessed
* add ga doc
* add fail message for ga
* Add convenience/clarity wrapper for Range initialization
* Test horizontal moves
* Add column jumping tests
* Add failing movement conditions for multi-word moves
* Refactor skip_over_next
* Add complex forward movement unit tests
* Add strict whitespace checks and edge case tests
* Restore formatting
* Remove unused function
* Add empty test case for deletion and fix nth_prev_word_boundary
* Add tests for backward motion
* Refactor word movement
* Address review comments and finish refactoring backwards move
* Finish unit test suite
* Fmt pass
* Fix lint erors
* Clean up diff restoring bad 'cargo fmt' actions
* Simplify movement closures (thanks Pickfire)
* Fmt pass
* Replace index-based movement with iterator based movement, ensuring that each move incurs a single call to the RopeSlice API
* Break down tuple function
* Extract common logic to all movement functions
* Split iterator helpers away into their own module
* WIP reducing clones
* Operate on spans
* WIP simplifying iterators
* Simplify motion helpers
* Fix iterator
* Fix all unit tests
* Refactor and simplify
* Simplify fold
