gh-89653: PEP 670: Convert unicodeobject.h macros to functions

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Convert unicodeobject.h macros to static inline functions:

• Reorder functions to declare functions before their first usage.
• PyUnicode_READ_CHAR() and PyUnicode_MAX_CHAR_VALUE() now only call
  PyUnicode_KIND() once.
• Simplify PyUnicode_GET_SIZE().
• PyUnicode_READ_CHAR() now uses PyUnicode_1BYTE_DATA(),
  PyUnicode_2BYTE_DATA() and PyUnicode_4BYTE_DATA().
• Remove redundant PyUnicode_Check() assertions.

Static inline functions are wrapped into macros which casts pointer
types (PyObject*, void*) to prevent introducing new compiler warnings
when passing const pointers (ex: PyUnicode_WRITE).

PyUnicode_KIND() return type is "unsigned int" rather than "enum
PyUnicode_Kind" to prevent introducing new compiler warnings.

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Changes since my #31221 POC:

• Add macros to prevent emitting new compiler warnings: so it's not needed to modify C code using these functions to fix const vs non-const => PEP 670 requires that
• PyUnicode_KIND() return type is now unsigned int, not enum Uncode_Kind
• The following functions return type is now unsigned int instead of int: PyUnicode_IS_COMPACT(), PyUnicode_CHECK_INTERNED(), PyUnicode_IS_READY(), PyUnicode_IS_ASCII(). Previously, the macros just exposed structure members of type unsigned int.
• Don't deprecate PyUnicode_GET_SIZE(), PyUnicode_GET_DATA_SIZE(), PyUnicode_AS_UNICODE(), PyUnicode_AS_DATA()
• Don't add new assertions to ease review.
• Don't convert _PyUnicodeWriter_Prepare() and _PyUnicodeWriter_PrepareKind() macros to functions to keep this PR as small as possible.

I will re-do some on these changes (like deprecate functions) as separated PRs once this PR is merged.

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Ah, and I also updated comments :-)

@erlend-aasland@gpshead: Would yo mind to review this PR?

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@erlend-aasland: Tell me if you prefer that I split this PR into smaller PRs.

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The following functions return type is now unsigned int instead of int: PyUnicode_IS_COMPACT(), PyUnicode_CHECK_INTERNED(), PyUnicode_IS_READY(), PyUnicode_IS_ASCII(). Previously, the macros just exposed structure members of type unsigned int.

Oh. It seems like it's wrong. The following code emits a new compiler warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare].

 int skind = PyUnicode_KIND(self); ... int rkind = skind; ... assert(PyUnicode_KIND(u) == rkind); 

[erlend-aasland]

@erlend-aasland: Tell me if you prefer that I split this PR into smaller PRs.

I would prefer that, yes :)

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I would prefer that, yes :)

Here is a shorter PR: #91705

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I splitted this PR into smaller PRs.