The entry point module for user-interfaces

This module holds the normal entry functions for the user interfaces
that use the calculator.
Basic interactive behaviour

Normal interactive behaviour implemented in the keypress-handling
function.
Added the engine module to the project

The interactive engine function.
Fixed a spot where a default was wrong

Fixed the default behaviour in the macro, which overwrote the default
in the structure's construction function.
Removed a field, changed a startup default

The calculator starts in :RUN-MODE-NO-PROG, and we no longer have a
separate rational mode.
Allow numbers to be passed as strings

Adjust the input code to permit numbers that are to be pushed onto the
stack to be sent in through the same interface as command codes.