* Changed the minimum supported version of Rust to 1.21.0 (semver breaking
  change).
* Flipped around the defaults vs. optional features for controlling the size
  vs. speed trade-off for Kanji and Hanzi legacy encode (semver breaking
  change).
* Added NEON support on ARMv7.
* SIMD-accelerated x-user-defined to UTF-16 decode.
* Made UTF-16LE and UTF-16BE decode a lot faster (including SIMD
  acceleration).