List index (subscript) operator, starting from 0. Returns
Nothing if the index is out of bounds
This is the total variant of the partial
!! operator.
WARNING: This function takes linear time in the index.
Examples
>>> ['a', 'b', 'c'] !? 0
Just 'a'
>>> ['a', 'b', 'c'] !? 2
Just 'c'
>>> ['a', 'b', 'c'] !? 3
Nothing
>>> ['a', 'b', 'c'] !? (-1)
Nothing