IndexRef is like a pointer, except it just stores a pointer to the container
and an index. Any invalid (out of range) IndexRef is equal to any other.
IndexIterator is an iterator over all IndexRef values for a container,
used to have a range-for loop where not only every loop,
but every access through the iterated value, gets looked up based
on index.

If this looks appealing I'll move the code out of the main
consuming CPP file into someplace where the other files can use it. It is coded so that the other places/excuses for not using range-for can also be accommodated (you can get the current index, and can also indicate you'd like to start at something other than element 0)

Since it provides pointer-like behavior using indices, this eliminates/prevents the entire class of bugs like #468: it makes the caching of a pointer redundant, so migrating to using this fixes it all without making the code look more noisy. (I only migrated one file's commented for-loops.)

(It can also be used without range-for in this way - less elegant, maybe, but it works:

        for(i = 0; i < SK.constraint.n; i++) {
            Constraint *cs = &(SK.constraint[i]);
            cs->DoStuff();
        }

becomes

        for(i = 0; i < SK.constraint.n; i++) {
            IndexRef<IdList<Constraint, hConstraint>, Constraint>  cs{SK.constraint, i};
            cs->DoStuff();
        }

We could simplify that type a good deal, but that's code that works now.)

I wish iteration/ranges/iterators didn't require quite so much noise to create, but in any case, that's what is required for now. Most confusing part of this is the middle level of iterator that doesn't actually point to the array, but instead just keeps track of an IndexRef and returns that instead of a pointer or reference.