gh-132983: Simplify _zstd_exec()

[AA-Turner]

cc @Rogdham (can't request review).

Follows on from #133670. Again best reviewed commit-by-commit. We remove around 100 lines of C code overall.

The last commit (6a3a135 (#133775)) only serves to merge two functions and contains no behaviour change, but does increase the diff size.

• Rename type specs to add underscores after 'zstd'
• Replace the add_type_to_module() function with an ADD_TYPE macro, using PyModule_AddType instead of PyModule_AddObjectRef.
• Remove the unused 'empty_readonly_memoryview', 'str_flush', 'str_read', 'str_readinto', and 'str_write' module state members
• Set the CONTINUE/FLUSH_* constants in Python rather than C. To achieve this, export the three ZSTD_EndDirective enum members from the _zstd module.

• Issue: Implement PEP 784 - Adding Zstandard to the Python standard library #132983

[picnixz]

Overall, I think it's a nice improvement but usually, C classes are read-only (e.g., _sha2.SHA2Type does not allow setting class variables afterwards; the same holds for lzma.LZMACompressor so it would be better to do the same for the Zstd* types)

[serhiy-storchaka]

It would be better to make classes implemented in C immutable.

[emmatyping]

Thanks! These are great refactors.

[picnixz]

You can add Py_TPFLAGS_IMMUTABLETYPE in this PR or in another.

[miss-islington-app]

Thanks @AA-Turner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-app]

GH-133786 is a backport of this pull request to the 3.14 branch.