Adds support for `for h : x in my_array do`, and relates this to the existing `List` version.
None of these were working previously: ``` #check_simp #[1,2,3,4,5][2] ~> 3 #check_simp #[1,2,3,4,5][2]? ~> some 3 #check_simp #[1,2,3,4,5][7]? ~> none #check_simp #[][0]? ~> none #check_simp #[1,2,3,4,5][2]! ~> 3 #check_simp #[1,2,3,4,5][7]! ~> (default : Nat) #check_simp (#[] : Array Nat)[0]! ~> (default : Nat) ```