v: if guard optional: Define err and errcode in else branch

[ntrel]

Also:

• cgen: Don't allocate guard_vars array until there's a guard
• parser: Use if sum is T instead of sum as T
• parser: Rename is_or variable to is_guard

[spytheman]

I think you need to also do p.scope.register('errcode', ast.Var{ here.

[ntrel]

Now done. I think we need a TODO for the docs that it's required to update when making undocumented changes, errcode is not documented. I'm also concerned that Option is too big to use as a true option type (i.e. without error fields). For that it should only be the size of a pointer if the element type is a pointer, setting the lowest bit when none, and not add more than a word for non-pointer element types.

[medvednikov]

It will be optimized. If error fields are not used, it will be a small type with just one extra boolean field.