Add EUC-TW for the whole-buffer and streaming decoders: ASCII, a two-byte
plane-1 pair, and a 0x8E four-byte plane selector plus a pair. CNS planes 1
and 2, which carry essentially all real-world EUC-TW text, are vendored from
the ICU euc-tw-2014 mapping (private-use fallbacks dropped); planes 3-15 are a
documented follow-up. Tested for plane 1 (two-byte and four-byte forms), plane
2, and streaming. The streaming method moved to its own file to stay under the
type-body cap.
Closes#108
