chore: rearrange bindings and entrypoints classes

[ryanking13]

This is my last code refactor PR:

• Remove workers/_worker.py
• Move bindings wrapper implementation to rpc.py
• Move entry points (DurableObjects, Workflows) to a separate entrypoint.py file

[ask-bonk]

The refactor is clean. All code has been properly moved, all imports updated, the circular import problem is solved by co-locating _BindingWrapper with python_to_rpc/python_from_rpc in rpc.py, and all tests reference the new paths. No code was lost, no exports are missing, and no stale references remain.

LGTM

github run