Add type annotations for structs.py

[RobertoRoos]

By the way, should we stick to the comment-style typing:

def func(foo, bar):
# type: (int, int) -> int
    return foo * bar

var = 5  # type: int

Or the more modern in-line style typing:

def func(foo: int, bar: int) -> int:
    return foo * bar

var: int = 5

Now both styles are being used.

[stlehmann]

We want to use type-annotations of the modern style alltogether.