avar2

fontc currently has no avar2 support. Here is an example of how to build avar2 tables in fontations given a set of input/output location mappings: https://github.com/simoncozens/babelfont-rs/blob/main/babelfont/src/convertors/fontir/avar2.rs

I'd provide a PR but I'm not sure how you're storing cross-axis mappings in your IR format, but hopefully this will be a useful reference point for implementation.