Rewrite string encoding conversion code

Rewrite string encoding conversion code

- Drop support for multibyte encodings other than UTF-8.  It is probably
  not worth the effort to support such encodings.  Interestingly, the
  support was entirely broken before v1.9.1, yet no one ever
  complained...

- Implement UTF-8 and UTF-16LE codecs and drop the iconv requirement.
  This provides better performance and actually results in *fewer* lines
  of code and a slightly smaller binary.