Set assumptions for TypedSymbol/cast_func/IntegerFunctionTwoArgsMixIn the SymPy way

After having a nearly week long discussion on assumptions in my SymPy PR, I got some idea of how the assumptions in SymPy are working.

It's interesting that you can use Function.__new__(cls, integer=True) for UndefinedFunctions like Function('f', interger=True) but not for subclassese of Function.

Now things like (2*f.shape[0]).is_integer are working.