This fixes a class of crashes and introduces ptr<T> to make the
code robust against this failure mode going forward.

Thanks regnat for the idea of a ref<T> without overhead! This became ptr<T>

Closes #4895
Closes #4893
Closes #5127
Closes #5113

Alternatives considered:

• &Pos: not mutable, so doesn't fit the current usage pattern
• ref<Pos>: undue overhead
• Just avoid null pointer dereference when attrSet->pos is null #4895 and Mitigate a segmentation fault when a derivation has no name attribute #5127: no simplification and no future guarantees. These do cover the most important changes, but it's sad because those could have been found automatically by the type system. At least it won't happen again for this issue.
• Remove noPos and use nullptr instead. This is fragile. Same for optional, which doesn't prevent this problem at compile time.