Add structured coroutines and explicit error handling

internal/lang/enum_test.go

@@ -49,8 +49,8 @@ fun use(result: Result) { match (result) { Result::Ok -> { println("ok") } } }`)
 
 func TestRejectWrongVariantPayloadCount(t *testing.T) {
 	prog, err := Parse(`package demo
-enum Result { Ok(String) }
-fun main() { val result = Result::Ok() }`)
+enum Outcome { Ok(String) }
+fun main() { val result = Outcome::Ok() }`)
 	if err != nil {
 		t.Fatal(err)
 	}