Hourglass
Implement the function
val hourglass : int -> string
so that hourglass n is a string in the shape of an hourglass with
2 * n + 1 lines.
Examples
let _ = assert (hourglass 0 = "*") let _ = assert (hourglass 1 = "***\n *\n***") let _ = assert (hourglass 4 = "*********\n" ^ " *******\n" ^ " *****\n" ^ " ***\n" ^ " *\n" ^ " ***\n" ^ " *****\n" ^ " *******\n" ^ "*********")