Support Reverse Polish Notation (RPN)

Problem Statement

Reverse Polish Notation / RPN / Postfix mode is an alternate calculator entry mode that is very powerful (leveraging a stack does this!) and should be simple enough to implement.

This behaviour can be seen in physical HP scientific calculators, or the calc mode in Emacs.

Evidence or User Insights

It's a simple addition with minimal overhead that would please some potential users that right now need to seek other solutions.

Goals

Users can toggle an RPN mode of entry
Low-Fidelity Concept

Simple psuedocode:

#! python3

# Or a function list, or something to check if input is "function" or "input"
functionMap = { 
    "+": lambda x, y: x + y,
    # ....
    "mod": lambda x, y: x % y
}

stack = []
while True:
    userInput = getInput()
    # If it's not a function, add it to the stack
    if userInput not in functionMap.keys():
        stack.append(userInput)
    else:
        # Evaluate the last two items on the stack.
        # In real usage, the last stack item is often implicit, eg, 
        # 27 [enter] 3 / 
        # rather than
        # 27 [enter] 3 [enter] /
        b = stack.pop()
        a = stack.pop()
        stack.append(functionMap[userInput](a, b))

Start with docs/NewFeatureProcess.md for the project's feature-pitch expectations, then review the RPN description and pseudocode in this issue. Done should mean users can toggle an RPN entry mode and use stack-based operands and operators as described; the issue does not name implementation files or tests.