Require KWargs for struct initialization

[cburgdorf]

What was wrong?

Example:

struct House:
    vacant: bool
    price: u256

Currently, House can be instantiated as House(true, 1000000).

This means:

1. Property assignment can easily get mixed up leading to bugs
2. struct initialization is visually very close to regular function invocations

How was it fixed?

With this change it is required to be instantiated like House(vacant=true, price=1000000)

While working on this it got me thinking if this is something that should be handled on the parser stage instead but I don't think that would be straight forward because technically there is nothing the syntax is identical with method invocation syntax.

We could move this into the parser stage if we used a distinct syntax such as House(vacant: true, price: 100). But I'm personally ok to keep it as with the check being handled by the analyzer.

[g-r-a-n-t]

Something I noticed while working on #189 is that initialization doesn't work much of the time, since the values are in an unpredictable order upon retrieval.

The following will fail to compile for me:

struct House:
    price: u256
    size: u256
    rooms: u8
    vacant: bool

contract Foo:
    pub def create_house():
        self.my_house = House(1, 2, u8(5), false)

as the attribute types just so happen to be retrieved in the the order u256, u8, u256, bool.

I'll fix this in my PR and we can go ahead and merge this.

[g-r-a-n-t]

fixed. I just added another attribute for key order to the struct struct