gh-132983: Reduce the size of _zstdmodule.h

[AA-Turner]

We should try and reduce the content in the header file, as it is exposed to anything that includes it.

• mt_continue_should_break() moves into compressor.c
• The decompress_type enum moves into decompressor.c
• Replace #pragma once (not used for any other modules or non-vendored code) with guard defines
• Use angle-bracket includes for z{std,dict}.h
• Add a zstddict.h header file to contain the definition of the ZstdDict struct
• Move the ZstdCompressor struct to compressor.c (likewise for decompressor)
• Make PyModuleDef _zstdmodule static

A

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

[hugovk]

Nit note about PR titles, the convention is lowercase gh-NNN for issues, and uppercase GH-NNN for PRs:

please make sure to reference the issue number using gh-NNNNN: prefix in the pull request title

https://devguide.python.org/getting-started/pull-request-lifecycle/#submitting

[3.9] gh-12345: Fix the Spam Module (GH-NNNN)

Here gh-12345 is the GitHub issue number, and GH-NNNN is the number of the original pull request.

https://devguide.python.org/core-developers/committing/#backporting-changes-to-an-older-version

[picnixz]

It's fun that the diff for reducing the size is +165/-164.

[miss-islington-app]

Thanks @AA-Turner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-133854 is a backport of this pull request to the 3.14 branch.