Add remove/modify functions for index/indices)

Patch provides 4 functions

* `modify-at`: replace element at index with new element
* `modify-with`: replace element at index with result of funciton `(a -> b)` with old element as input
* `remove-at`: remove element at index (how come this didn't exist yet, after a year! :D)
* `remove-at-indices`: remove elements at indices.