Types of `Map.get` and `BigMap.get` in SCaml.ml are incorrect

In src/tests/SCaml.ml, Map.get is

  let get : 'k -> ('k, 'v) t -> 'k option = fun _ -> assert false

but it should be

  let get : 'k -> ('k, 'v) t -> 'v option = fun _ -> assert false

(the type of return value is incorrect).

BigMap.get is the same.