One native binary per architecture-specific gem

[djudd]

Currently I've only got cross-compilation of Rust set up on my laptop and so the process is: rebuild all the libraries, then check them in (yuck), then push that same code to every platform. It'd probably be better to do the Rust builds in Github Actions and push the native code only with the appropriate platform-specific gem.

Blocked on #8 and optionally #9 (the alternative to the latter would probably be to drop support for arm64 outside Mac, and drop support for x86 Mac).