Provide a macro-annotation to reduce defining Encoder/Decoder pairs about circe

[travisbrown]

I've been resisting adding macros to circe, but the seal is broken now, so I think we should consider this.

We'd have to change the language in the first item in the design doc a bit. The prohibition against annotations there is primarily intended to target member-level annotations, though, and I don't think this kind of thing (where the macro annotation is just nice syntax for semiauto definitions) is so bad.

We'd probably want something like @ConfiguredJsonCodec[C] in addition to @JsonCodec (or whatever they end up being called).

Are you interested in adding this, @dwijnand? I'm not sure I'd be able to get to it for a week or so.

[dwijnand]

I'm not against rolling up my sleeves and contributing, but I've a stack of sbt things I'd like to tend to, so I don't think I'll be able to get to it in a week or so.

[ngbinh]

This is something we love to have also.

[dwijnand]

Initial support for this is now in master (via #203).

Support for a proposed @ConfiguredJsonCodec[C] one could say is blocked on still pending #164.

[dwijnand]

I should rivisit the tail part of this given #429 is merged.

[travisbrown]

#429 includes @ConfiguredJsonCodec so I think we can close this.