bpo-45490: Rename static inline functions (GH-31217)

When a static inline function is wrapped by a macro which casts its
arguments to the expected type, there is no need that the function
has a different name than the macro. Use the same name for the macro
and the function to avoid confusion.

Rename _PyUnicode_get_wstr_length() to PyUnicode_WSTR_LENGTH().

Don't rename static inline _Py_NewRef() and _Py_XNewRef() functions,
since the C API exports Py_NewRef() and Py_XNewRef() functions as
regular functions. The name cannot be reused in this case.