add string-explode

[puripuri2100]

This PR adds string-explode: string -> int list.

string-explode converts string to Unicode code point sequence.

ex.

  string-explode `SATySFiは組版ソフトです`
    => [83; 65; 84; 121; 83; 70; 105; 12399; 32068; 29256; 12477; 12501; 12488; 12391; 12377]
  string-explode `SATySFiは組版ソフトです` |> string-unexplode
    => SATySFiは組版ソフトです

[gfngfn]

I am sorry for the late response. Thank you for improvement!