Add the WHATWG big5 decoder over the vendored index-big5 (18,590 entries),
for both the whole-buffer and streaming decoders. A lead in 0x81-0xFE plus
a trail in 0x40-0x7E or 0xA1-0xFE resolves through the index, covering
Traditional Chinese and the HKSCS extensions including the astral CJK
ideographs. Four pointers decode to a base letter plus a combining mark.
Selected by the big5 / big5-hkscs declaration names.
Closes#103
