GB18030: Help transitioning away from PUA code points.

* lib/gb18030ext.h (gb18030_2005_ext_wctomb): Remove function.
(gb18030ext_wctomb): Renamed from gb18030_2022_ext_wctomb.
* lib/gb18030uni.h (gb18030_2005_uni_wctomb): Map 6 Ext-B code points to
4-bytes sequences.
(gb18030_2022_uni_wctomb): Small refactoring.
* lib/gb18030_2005.h (gb18030_2005_pua2charset): Map 6 PUA code points
to 4-bytes sequences instead of 2-bytes sequences.
(gb18030_2005_wctomb): Update accordingly. Invoke gb18030ext_wctomb
instead of gb18030_2005_ext_wctomb.
* lib/gb18030_2022.h (gb18030_2022_wctomb): Invoke gb18030ext_wctomb
instead of gb18030_2022_ext_wctomb.
* tests/GB18030-2005.IRREVERSIBLE.TXT: Update the inverse mappings of 6
Ext-B code points and 6 PUA code points.
* NEWS: Mention it.
7 files changed