Chapter 3: Use of Deque to implement RpnStack

[isben]

In Chapter 3, implementation of the RpnStack is done using a Deque.
While the code is technically correct and properly implements the RpnStack, it is not strictly required.
Replacing the Deque by a simple Vec in the code produces the same result. push and pop operations of a Vec behave exactly as you would expect from an usual stack, hence no need to over complicate things by introducing a Deque