Full support for pub qualifier on structs

[cburgdorf]

What is wrong?

When #203 lands, we'll have basic support for structs.

struct House:
    price: u256
    size: u256
    pub vacant: bool

Notice that vacant uses pub to declare that the field is public. We know that we want to allow pub qualifier which will become most relevant for when structs are allowed to have methods. This is why the parser does already recognize it. However, it doesn't have any effect yet.

How can it be fixed

1. Define how visibility of struct fields should work exactly
2. Make the relevant changes to analyzer and mapper pass