the type casts we were using were causing bad things to happen with gcc builds (still not exactly sure
why they aren't in clang builds), since we were casting from a function which expects arguments on the stack
to a function type that doesn't.

this manifests itself as rewrite_args changing from NULL to a small heap pointer on this line:

```
objmodel.cpp:4108    contained = callattrInternal1(rhs, contains_str, CLASS_ONLY, NULL, ArgPassSpec(1), lhs);
```

that is, the local variable rewrite_args is NULL before the call, and non-NULL after.  The actual line that causes
the pointer overwrite is in `bindObjIntoArgs`:

```
objmodel.cpp:3043    arg1 = bind_obj;
```

so I'm guessing that since we didn't push things onto the stack before the call to `callattrInternal`, we end up
trampling over values in `compareInternal`'s frame.