Allow assigning untyped objects to struct properties

[benStre]

Currently, nested structs can be created with normal objects, e.g.

const example = MyStruct({
  name: 1,
  address: {
    street: "xy"
  }
})

But when assigning a new object to a property, an error occurs:

example.address = {street: "z"}; // Error: address must be of type <MyAddress>

It should be possible to assign objects without explicitly converting them to the matching type beforehand.