Rename Transcoder and move some state up in prep for porting.
Switch over to using jcodings for transcode.
Reimpl a bit of gsub pipeline to match MRI better.
Start wiring Converter up to new transcoder.

* convpath logics implemented properly now.
* Most other methods still to be converted.
Bits and bobs of ported code for transcoding.
Get most of Converter converted over to new transcoder.
Implement String#scrub (issues remain in jcodings).

* From my impl (modified) at rubinius/rubinius#2912
* Fails due to ArrayStoreException in jcodings