this should make the whole decoder business easy to maintainable.
currently this change is available to Korean and Japanese codecs.

this also keeps up with the recent WHATWG spec changes, including
a merge of GBK18030 with GB18030.

previously it didn't depend on the encoder state at all, resulting in
the incorrect output when the encoder has a complex state.
this directly affects ISO 2022 encodings in the development.

…decoders.

- the last codec error is now stored in the intermediate context,
  so the state machine can use a code like `ctx.err(..), S1(ctx)`.
- fixed a performance regression due to the decoder generalization.
  the decoder performance should be on par with hand-written one
  (less than 10% difference) with `--opt-level=3 -Z lto` rustc flags.

this also tries to fix a possible interference issue between
the benchmark and aggressive optimization passes.

…mpls.

this greatly improves backward mapping performance, with improvements
ranging from 50% to 400%. this also allows for the exact accounting of
the binary size overhead.

Depends on servo/servo#2472.

Rebuild when external dependencies change

this also changes names of Macintosh encodings from `encoding::all`
to stay away from names assigned by WHATWG Encoding Standard.