Indeed, the FFI in Nim is really great, and with ARC it's so much easier to properly integrate with the malloc/free lifecycle that most C libraries use. The full wrapper for this project is here:
https://github.com/PMunch/mapm-nim and I've already converted a calculator program of mine to using this instead of floats by simply just swapping out the type. Checked with Valgrind and besides finding some bugs in another external library it was happy.