RFC: Index updates

* Define `show`, `showall` for `Index`
* Bugfix: `rename!` needs to return the modified `Index` for `rename` to work
* Added `pop!`, `shift!`, `unshift!`, `append!`, `prepend!`
* Bugfix: `delete!` should return the deleted item (for parity with mainline julia, use by `pop!`, `unshift!`)
* Bugfix: `push!` should return the modified `Index` (for parity with mainline julia)
* Added tests for the above