sandbox_{stack,heap}arr_helper could use some useful methods/operators

[froydnj]

Doing anything safely with these types besides passing them into sandboxed functions is difficult, because they don't have useful operators (e.g. operator* for pointers), especially compared to their tainted<T> counterparts. Using them would be preferable to raw mallocInSandbox operations because we can ensure they are cleaned up at the end of their containing block.