Add destructuring

[rpip]

Some examples of destructuring

1. let {x,y,_} = {1,4,5}

let guess_type =  fn x ->
  case x do
      1 ->
          :number;
      3..10 ->
          :range;
      x::xs ->
          :list;
      {name = _} ->
          :record
  end
;;

x::y = [1,2,3] results in x = 1, y = [2,3]

x,y::xs = [1,2,3] results in x = 1, y = 2, xs = [3]

x,y::_ = [1,2,3] results in x = 1, y = 2