Implement Index<Range<…>> for OpStack

[jan-ferdinand]

The operational stack can be indexed into to fetch individual elements through its implementations of Index<usize> and Index<OpStackElement> (and the corresponding IndexMut<…>). It can be useful to get a slice from the op stack.

Implement the combinations {Index, IndexMut} $\times$ {Range, RangeInclusive} $\times$ {usize, OpStackElement} for OpStack.

[jan-ferdinand]

#338 has uncovered that there are some rather fundamental issues when trying to add this feature. Summarizing for posteriority:

Currently, OpStack uses a Vec under the hood. The top of Triton VM's op stack, or op_stack[0], is the last element in the internal vector. To provide semantically correct access to some op_stack[i..j] range, the underlying vector has to be indexed “in reverse”, which does not produce a contiguous slice.

It is possible to allocate a new vector containing the correct stack elements and to then leak that new vector. I feel uncomfortable creating memory leaks like this.

Using a VecDeque, which seemed like the obvious next choice, also does not work, because it is implemented as a ring buffer. This means that a range of elements from a VecDeque are not necessarily contiguous in memory, requiring re-organization within the implementation of the Index trait again, leading back to the same problems the current implementation is facing.

The only real solution I can see is to implement a “reverse” vector, such that op_stack[0] is the always the first element in that data structure.