RPN Calculator face (#112)

RPN Calculator face (#112)

* Calculator face with guess a number

* Add more functions, cleanup, and blinking

* Display operations better in the first location

* Clean up stack behaviour

Add empty stack behaviour, check stack bounds, add stack operations.

Also, make 'mode' always work to switch through faces on entering calculator
(i.e. by defaulting to OPERATION rather than NUMBER).

Some edge cases.

* Tweak negative rounding behaviour; add basic doco

* Add copyright name.

* Flip ordering of binary operations

* Fix compiler warnings

* Remove watch_private_display hackery and give face a better name

Also, slightly clearer documentation and a dupe function.