APx is finally available on crates.io / docs.rs
https://docs.rs/apx_sdk/latest/apx_sdk/
This is an ActivityPub library used internally in Mitra and in other my projects.
Unlike some other ActivityPub libraries, this library is low-level. It doesn't restrict what databases or frameworks could be used, and doesn't care about object types or properties. There are only URIs, keys, signatures, JSON documents and HTTP requests.
It also contains primitives for building nomadic applications (FEP-ef61).
The license is AGPL-3, but I might consider changing it to a more permissive one.