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

[Dave Britten]

Somehow I missed the discussion of this when it came up a few months ago. Definitely going to load this up on my 83+ Silver and 84+ Pocket. Adding RPN, base conversions, and a few unit conversions could actually push the 83/84 into daily-driver territory for me. It looks like there's a decent selection of unit-conversion functions, but my one request would be to make it as complete and flexible as the 19BII, TI-85, etc.