Improves UTF-8 decode performance and single-byte encode performance.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=69955b3af3d1399433479453979dcb66db7d728d Note that most of the churn in the diff is due to: * Legacy CJK encoder-specific compile-time options that aren't enabled by Firefox. * Addressing clippy lints. * rustfmt keeping changing formatting. Looking at the individual changesets on GitHub probably gives a better idea of the changes than the diff from release to release: https://github.com/hsivonen/encoding_rs/commits/master

0.8.12 fixes clippy compatibility on stable Rust.

* Improves UTF-8 validation performance. * Improves UTF-8 to UTF-16 decode performance. * Improves non-Latin and Latin1-ish Latin single-byte encode performance. * Improves code quality by addressing some clippy lints. The optional legacy CJK encoder changes are not used by Firefox.

Pushed by hsivonen@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/3691033d6e78 Update encoding_rs to 0.8.12. r=m_kato