RPN scientific calculator firmware for $13 calculator kit

[jklsadf]

I have not made much progress recently, what's on github is fairly up to date. The last change I made was some prototyping work for replacing the existing square root implementation (using logs/antilogs) with a fast-reciprocal square root implementation. I haven't done the implementation for the calculator yet though. The motivation was to reduce the decimal floating point register usage for square root, to use it as part of trig algorithms like those used by valentin albillo for the HP 12C.

I do still use this calculator regularly (it has pretty much a permanent place on my desk at home), but haven't spent much time on it recently. I do want to get trigs done, but probably not much flash left for much else...even for the trigs will probably need to rewrite some other parts to free up some space, so it hasn't been that motivating given how rarely I use trigs.