checker: add error when initializing sumtype with struct as first type

[felipensp]

This PR add a checker error when initializing struct via sumtype as first variant type.

struct Node {
    parent &Node
    left &Node
    right &Node
}
type Sumtype = Node | int

fn main() {
    s := Sumtype{}
    dump(s)
}

bug.v:9:10: error: reference field `Node.parent` must be initialized
    7 | 
    8 | fn main() {
    9 |     s := Sumtype{}
      |          ~~~~~~~~~
   10 |     dump(s)
   11 | }
bug.v:9:10: error: reference field `Node.left` must be initialized
    7 | 
    8 | fn main() {
    9 |     s := Sumtype{}
      |          ~~~~~~~~~
   10 |     dump(s)
   11 | }
bug.v:9:10: error: reference field `Node.right` must be initialized
    7 | 
    8 | fn main() {
    9 |     s := Sumtype{}
      |          ~~~~~~~~~
   10 |     dump(s)
   11 | }

[spytheman]

Excellent work.