Add a type class combining encoding and decoding about circe

[tpolecat]

Also if you add Codec[A] you might consider whether the more general profunctor GenCodec[A,B] (name stolen from scodec) is interesting enough to keep around. It kind of pops out. I have it in doobie for now but I'm kind of on the fence.

[tpolecat]

FWIW I just went through this in reverse with doobie (and had a good discussion with @mpilquist who went through a similar exercise with scodec). My approach is:

1. All code is written in terms of Decode and Encode; if you need both you ask for both.
2. Codec is just a convenience for the common case where you have a symmetric pair and want to define them together (or xmap an existing pair). It's not used as a context bound.
3. Encode and Decode have implicit derivations that consume and return a Codec to fix the search problem described above.
4. You can't get a Codec implicitly from an Encode + Decode. I think this turns out to not be very useful because of (1).

In my case you almost always have read/write pairs so getting rid of Codec would be really inconvenient.

[travisbrown]

Done in #1151.