various refactorings:

- establish a common interface for encoders and decoders, and
  re-implement the existing functions accordingly
- encoders always convert STRING_LIST to BLOB, decoders vice versa
- VMOD encode() and decode() write the new data to workspace
- transcode() writes the temporary BLOB to stack, then encodes it
  into a STRING in workspace
- if the same encoding is specified in transcode() for encode and decode,
  just return the concatenated STRING
  - XXX: except this won't reject illegal decodings
- simplify hex_encode(), and fix hex_en-/decode_l() (was backwards)
- add a macro to skip over empty and undefined strings in a STRING_LIST