Arguments in function types should be contravariant

[JukkaL]

Currently function types are invariant with respect to argument types. This makes it impossible to use some useful and common idioms, for example:

print(list(map(str, [1, 2, 3])))

Function subtyping (and type inference, if needed) should be modified so that the above code works.

However, argument types should perhaps still be invariant in overriding to avoid complications with overloading.