_PyCoreConfig: Change filesystem_encoding, filesystem_errors,
stdio_encoding and stdio_errors fields type from char* to wchar_t*.
Changes:
* PyInterpreterState: replace fscodec_initialized (int) with fs_codec
structure.
* Add get_error_handler_wide() and unicode_encode_utf8() helper
functions.
* Add error_handler parameter to unicode_encode_locale()
and unicode_decode_locale().
* Remove _PyCoreConfig_SetString().
* Rename _PyCoreConfig_SetWideString() to _PyCoreConfig_SetString().
* Rename _PyCoreConfig_SetWideStringFromString()
to _PyCoreConfig_DecodeLocale().