`Array.events` spuriously synchronizes on read-after-read

- [ ] There should be `Array.write_events` and `Array.read_events`, with `Array.events` aliasing `write_events` under a deprecation.
- [ ] Arguments to all the generated kernels (elementwise etc.) should declare whether they're writing and automatically add to the appropriate `events` list.
- [ ] Audit code calling into the kernels to avoid duplicate event insertion.

cc @alexfikl @zachjweiner