Immutable Reference Implementation

[mjp41]

This is the top-level issue for tracking progress for making the reference implementation in line with the in progress draft PEP.

The following task should be expanded into issues.

• Add self contained check to freeze (After meeting decided against this approach.)
• Add SCC and atomic RC to allow cross interpreter object management
• Add support to interpreter communication to allow frozen objects to be transferred without pickling.
• Separate immutable module into _immutable for C components of immutable module, and immutable for the parts that can be written in Python.
• Add a pre-freeze hook.
• Expose a tp_references that walks the object graph without avoiding objects that cannot participate in cycles. tp_traverse has some optimisations that mean it doesn't walk all the reachable objects.
• Provide register_shallow_freezable to allow C modules to specify that the types are immutable by construction.
• Correctly handle getting the module state from a type to access the state associated with the current interpreter
• Correctly handle subclasses access with locks and copying
• Escape hatch for thread-local/interpreter-local
• Escape hatch for shared cell that contains only immutable data

This issue should be update and sub-issues created to track the outstanding work.