Character encoding and string conversion updates

- Allow unpaired surrogates when translating between "UTF-8" and
  "UTF-16LE".  This allows Windows-style filenames to be processed
  losslessly on UNIX-like systems, even if they are not valid UTF-16LE.

- Implement UTF-8 and UTF-16LE codecs ourselves and drop the iconv
  requirement.  This was necessary to allow surrogate codepoints, but it
  also provides better performance and actually results in *fewer* lines
  of code and a slightly smaller binary.

- Drop support for multibyte encodings other than UTF-8 on UNIX-like
  systems.  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...