Feature Request: Tensorflow Stack Ops

[jthestness]

Tensorflow stack ops track activations for backpropping while loops. These ops maintain a stack of tensor handles that point to activations for each iteration of the while loop. We need to add support for StackPush(V?) and StackPop(V?) ops to Catamount.

These ops will require the following functionality:

1. Since StackPush ops need to execute before corresponding StackPop ops, we may need to implement ordering requirements (e.g., control dependencies) to ensure topological traversals consider the pushes before the pops
2. The data pushed in a StackPush needs to be returned in the StackPop. Corresponding ops may need to manage a backing stack of tensors to ensure we can track the shapes and values as appropriate.