Refactor Xairo into helper modules

[mikowitz]

It's getting huge, and maybe a bit unwieldy.

Break functions out into reasonably scoped modules (mirroring the Rust divisions?) that can hold documentation and defdelegate everything from Xairo? Keep the overall docs in there though, and make sure they stay up to date.

Is that worth it for an end user? Or is this really just about development ease?