bug: OS addition results in Int(0) for dest_ptr

[notlesh]

We previously created a workaround for an issue where dest_ptr is not a relocatable in #371. This PR works around the issue, but doesn’t understand or fix it.

I spent some more time looking at this recently and wanted to summarize my findings. I was not able to reproduce the issue with a standalone cairo0 program. It seems the compiler is good about preventing pointers-to-structs from being cast as integers in the way we see in the original issue.

The issue seems to happen when we add 1 to the pointer, that is (from segment_arena.cairo):

• let dest_ptr = infos[0].end + 1; ⇒ dest_ptr is Int(0)
• let end_ptr = infos[0].end;
let dest_ptr = end_ptr + 1;
  ⇒ end_ptr is a proper relocatable, dest_ptr is Int(0)
• While experimenting with cairo0 code, I found that a ptr to a struct gets a special Python treatment; specifically, it is represented as an instance of starkware.cairo.lang.vm.vm_consts.VmConstsReference. This may impact the way hints are executed (for example, it may have special treatment for the + operator).
• There are no relocation rules present at the time this code is executed.

Given the above, I think there is something unexpected happening during the addition.