MPD.Internal: redo encoding

- Remove encodeUtf8 from recv; printing plain bytestrings
  containing special characters appears to work fine.
- Fix pack so that it actually works for non-ASCII characters.
  Now we can do `simpleMPD_ ["search artist \"La Düsseldorf\""]` and
  get results, whereas the old implementation would simply crash with
  a decoding error.

There are probably issues lurking here, but that is for someone who
actually knows how this stuff works to figure out (if such a person
should happen to care enough to look into it).