parser: allow multi returns in high order fns

compiler/tests/fn_test.v

@@ -107,6 +107,14 @@ fn high_fn(f fn(int) int) {
 
 }
 
+fn high_fn_array(f fn(a []int) []int) {
+
+}
+
+fn high_fn_multi_return(a int, b fn (c []int, d []string) ([]int, []string)) {
+
+}
+
 fn test_fns() {
 	// no asserts for now, just test function declarations above
 }