Struct allocation with alloc should initialize default fields

[raoulvdberge]

No description provided.

[raoulvdberge]

This is a little more complicated than I thought. Since it's an expression, this should work in every context: return, arguments in functions, ...

Since C cannot handle struct initializers without assigning, I'm not sure if we're able to do this in a clean way.

[MovingtoMars]

Yeah, it's kinda difficult. That's why I didn't have it in the original PR.

One way it could be done is by creating functions like this:

__alloc_Test(): Test {
    ret: Test = alloc Test;
    ret->x = 5;
    return ret;
}

and using them in the codegen.

[felixangell]

In #345