Any way to do a binary tree traversal without `opApply` and allocations?

[Richard (Rikki) Andrew Cattermole]

I saw on Discord that the issue with the existing opApply was you were 
hard coding the return values (which was never correct).

If that is not the only issue please let me know.

As for how to do the opApply attribute handling, here is how I do it: 
https://github.com/Project-Sidero/basic_memory/blob/main/generated/sidero/base/internal/meta.d