Typing system struggles to extend global objects

[71104]

The current implementation in 1cf6592 (HEAD of types branch) doesn't play well when trying to extend variables that are not explicitly defined by the program:

Math.pow                      # closure: unknown

let Math.lol = 0 in Math.pow  # type error

The type system can't possibly tell whether a variable is provided by the native environment or not.

Possible solution: make unknown an extensible type by making UnknownType actually inherit UndefinedType.

It would also be good to remove all uses of is and isAny in types.