Split Codecs into Encoders and Decoders

[pickypg]

Related to #1833, Codecs should be split into Encoders and Decoders to avoid some false assumptions and therefore semi-trappy behavior.

This would also make it much easier for plugins to clarify what they will handle (e.g., json via an decoder, but since there would not be an equivalent decoder for rubydebug, then obviously it would not work).