RPN83P: RPN calculator for TI-83+ TI-84+ inspired by HP-42S

[ijabbott]

I've just tried out version 0.7, but haven't tried earlier versions yet. I couldn't get the SHOW (2ND ENTRY) function to work. Perhaps it got broken on this version?

Minor suggestions:

• 2ND ( could be used for ROLL UP.
• 2ND ) could have the same effect as ) ) (X⇄Y X⇄Y), effectively terminating digit entry, similar to the ON/EXIT key on HP-42S. (Maybe not worth it as ) ) would be easier to press.)
• If it is easy to do, could the leading 0 of decimal fractions between 0 and 1 be shown in fixed/normal mode (for example "0.1" rather than ".1")? (I suspect it isn't shown because the TI-83/84 routines do not show it either!)