Readability: use std::stack in lieu of Cons

[dabrahams]

Per @jsiek. It will help readability to see the words stack, push, and pop in the code where that's the actual abstraction at work.

[jsiek]

To update this issue. Some uses of Cons were replaced by a new Stack class. The next step is to replace the Stack class with uses of std::stack.

[dabrahams]

I am not yet convinced using std::stack is a good move. See the last part of #327 (comment) for details.

[jonmeow]

I think the Stack switch is the direction we're going, I'm considering this resolved.