API changes
- Most optional parameters of primitives (e.g. `comm` or `token`) are now keyword-only
- Removed deprecated upper-cases aliases (e.g. use `send` instead of `Send`)
New features
- New primitive `barrier`
- Printing of MPI error strings on error
- `sendrecv` is now differentiable
Bug fixes
- Primitive outputs are now always tuples, not lists
- Debug messages are flushed immediately