gh-125063: marshal: Add version 5, improve documentation

[encukou]

This builds on #126804, but I followed the rabbit hole much deeper.

In Python 3.14, slice objects can be marshalled.
This calls for a new version of the marshal format, so that backwards-incompatible
objects are rejected at serialization time.

• Bump marshal version to 5
• Reject slices in earlier versions
• Reorganize the TYPE_ defines to make it easier to contribute
• Adjust and improve the documentation.

• Issue: Emit slices as constants in the bytecode compiler #125063

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📚 Documentation preview 📚: https://cpython-previews--126829.org.readthedocs.build/

[encukou]

In the Makefile, _freeze_module doesn't depend on marshal.h, but it needs to be rebuilt when Py_MARSHAL_VERSION switches to 5.
Instead of properly changing the Makefile, which would be another rabbit hole, I decided to touch _freeze_module.c instead.

[mdboom]

Looks good to me. I had no idea changing marshal required all this, so I appreciate the backfill.