gh-89653: PEP 670: Convert unicodeobject.h macros to functions (#91705)

Convert unicodeobject.h macros to static inline functions:

* PyUnicode_CHECK_INTERNED()
* PyUnicode_DATA(), _PyUnicode_COMPACT_DATA(),
  _PyUnicode_NONCOMPACT_DATA()
* PyUnicode_GET_LENGTH()
* PyUnicode_IS_ASCII()
* PyUnicode_IS_COMPACT()
* PyUnicode_IS_COMPACT_ASCII()
* PyUnicode_IS_READY()

Reorder functions to declare functions before their first usage.

Static inline functions are wrapped by macros which casts
arguments with _PyObject_CAST() to prevent introducing
new compiler warnings when passing "const PyObject*".