Replace messy _Py_wstrlist_xxx() functions with a new clean
_PyWstrList structure and new _PyWstrList_xxx() functions.
Changes:
* Add _PyCoreConfig.use_module_search_paths to decide if
_PyCoreConfig.module_search_paths should be computed or not, to
support empty search path list.
* _PyWstrList_Clear() sets length to 0 and items to NULL, whereas
_Py_wstrlist_clear() only freed memory.
* _PyWstrList_Append() returns an int, whereas _Py_wstrlist_append()
returned _PyInitError.
* _PyWstrList uses Py_ssize_t for the length, instead of int.
* Replace (int, wchar_t**) with _PyWstrList in:
* _PyPreConfig
* _PyCoreConfig
* _PyPreCmdline
* _PyCmdline
* Replace "int orig_argv; wchar_t **orig_argv;"
with "_PyWstrList orig_argv".
* _PyCmdline and _PyPreCmdline now also copy wchar_argv.
* Rename _PyArgv_Decode() to _PyArgv_AsWstrList().
* PySys_SetArgvEx() now pass the fixed (argc, argv) to
_PyPathConfig_ComputeArgv0() (don't pass negative argc or NULL
argv).
* _PyOS_GetOpt() uses Py_ssize_t
* All internal header files now require Py_BUILD_CORE or
Py_BUILD_CORE_BUILTIN to be defined.
* _json.c is now compiled with Py_BUILD_CORE_BUILTIN to access
pycore_accu.h header.
* Add an example to Modules/Setup to show how to build _json
as a built-in module; it requires non trivial compiler options.
Victor Stinner