Polymorphic types

	TV "a"				'a

	Fun (TV "a", TV "b")		'a -> 'b

	Lambda ("x", Fun (TV "a", TV "b"), Var "x")
	: TypeSchema (["a"; "b"], Fun (Fun (TV "a", TV "b"), Fun (TV "a", TV "b")))

	fun (x : 'a -> 'b) -> x : ('a -> 'b) -> ('a -> 'b)