Add an exported symbol

The UIs will need to know how to format strings for display.
Basic event loop and operations working

Code now in place to play an event loop, display the stack, and
perform operations on numbers.  The beginning of a calculator.
Change logic so UI doesn't have to determine number-edness

We had a keyword that could say that a given string was to be
interpreted as a number, for the CLI.  Rather than making the CLI
examine each string before it hands it down, we change the logic here
to say that if a string has arrived with that flag set, and the string
isn't a number, allow it to fall through to the normal keypress handling.