or statement for sum type instances; if guard optional syntax

[ntrel]

If we don't want #5835, there are 2 simpler things we can do to help make sum type handling and optional handling more similar:

1. if val := opt { was only documented last week in doc: Document optional if val := opt { #5838. We could change the syntax to use as for consistency with if sum is T as var and match sum as var:

if opt as val {

2. or for sum type instances:

var := sum is T or {