Over the holiday I was inspired to create a big number library for imp Squirrel - https://github.com/deldrid1/big.nut
It’s a fork of a JS library and nearly all the basics and up and running (and tested!). I’ve only been using it on the agent so far but it should work on the device (although at what level of performance and amount of code space I have no idea).
I’m sure there are some much more efficient implementations out there but its hard to beat “working code”. Feedback, comments, questions all appreciated!