Add String.sliding / chunksOf

[Dean177]

Array has the sliding and chunksOf functions:

val sliding : ?step:int -> 'a t -> size:int -> 'a t t
   (** Provides a sliding 'window' of sub-arrays over an array.

     The first sub-array starts at index [0] of the array and takes the first [size] elements.

     The sub-array then advances the index [step] (which defaults to 1) positions before taking the next [size] elements.

     The sub-arrays are guaranteed to always be of length [size] and iteration stops once a sub-array would extend beyond the end of the array.

     {[Array.sliding [|1;2;3;4;5|] ~size:1 = [|[|1|]; [|2|]; [|3|]; [|4|]; [|5|]|] ]}

     {[Array.sliding [|1;2;3;4;5|] ~size:2 = [|[|1;2|]; [|2;3|]; [|3;4|]; [|4;5|]|] ]}

     {[Array.sliding [|1;2;3;4;5|] ~size:3 = [|[|1;2;3|]; [|2;3;4|]; [|3;4;5|]|] ]}

     {[Array.sliding [|1;2;3;4;5|] ~size:2 ~step:2 = [|[|1;2|]; [|3;4|]|] ]}

     {[Array.sliding [|1;2;3;4;5|] ~size:1 ~step:3 = [|[|1|]; [|4|]|] ]}
   *)

let chunksOf t ~size = sliding t ~step:size ~size

These would be good additions to the String module too