Immutability Implementation Adjustments for PLDI and the PEP

[xFrednet]

• Add new tp_reachable (same signature as tp_traverse) to PyTypeObject (Needed for the PEP not PLDI)
• Split immutable module into a Python module and a C module
• Add @freezable, @frozen, @explicitlyFreezable and @unfreezable decorator
• Add set_freezable() with the values YES, NO, EXPLICIT and PROXY(For modules) to the immutable module
  • Implement the Explicit freezability
  • Check for freezability in freezing code
• Allow freeze() to take multiple arguments
• Make freeze() return the (first) passed in object
• Remove old code from the immutable module, for example immutable_register_freezable
• Add ImmutableError to the immutable module (Error name and message is open to change)
• Modules & Types should be unfreezable by default
  • PLDI specific: Modules should have the Proxy immutability by default or different default should be documented
  • Investigate if _PyOwnership_is_c_wrapper is still needed
• Make shadow_function_globals less smart (Don't freeze values based on strings which might index)
• Add escape hatches (§6.2):
  • Sub-interpreter Local
  • Cell type
• Add mutable(obj, ...) context manager to temporarily set freezability to No
  • Maybe add a C mechanism which could be used in a similar way
• Investigate naming convention, should isfrozen be is_frozen?
  • Either adjust paper (which uses is_frozen)
  • Or our implementation which uses isfrozen
• Support immutability by construction (§5.3 in the paper)
  • Add register_shallow_freezable(𝑡𝑦𝑝𝑒)
  • Make these objects immutable on first observation
• Some test were failing for module objects in proxy mode
• Expose mutable module state in sys.mut_modules
• Add pre-freeze hook (and check for it)
  • Change module and functions to use the pre-freeze hook instead of if-statement

Pending TODOs, but not critical for PLDI or DPO:

• Improve RC overhead for frozen objects