tractor

This is your first big boi, “from GH issue” design, plan and implement task.

We need to try and add sub-interpreter (aka subint) support per the issue,

Part of this work should include,

modularizing and thus better organizing the .spawn.* subpkg by breaking up various backends currently in spawn._spawn into separate submods where it makes sense.
add a new ._subint backend which tries to keep as much of the inter-process-isolation machinery in use as possible but with plans to optimize for localhost only benefits as offered by python’s subints where possible.
- utilizing localhost-only tpts like UDS, shm-buffers for performant IPC between subactors but also leveraging the benefits from the traditional OS subprocs mem/storage-domain isolation, linux namespaces where possible and as available/permitted by whatever is happening under the hood with how cpython implements subints.
- default configuration should encourage state isolation as with subprocs, but explicit public escape hatches to enable rigorously managed shm channels for high performance apps.
all tests should be (able to be) parameterized to use the new subints backend and enabled by flag in the harness using the existing pytest --spawn-backend <spawn-backend> support offered in the open_root_actor() and .testing._pytest harness override fixture.