Add Encoder/Decoder for Nothing

[LukaJCB]

Ran into a problem with an ADT with multiple type parameters that was due to there being no Nothing codec in circe. This may be controversial, but here's a motivating example:

sealed trait ADT[+A, +B]

case class Foo[+A, +B](val a: A, val b: B) extends ADT[A, B]
case class Bar[+B](val b: B) extends ADT[Nothing, B]

implicit def adtEncoder[A: Encoder, B: Encoder]: Encoder[ADT[A, B]] = ...
implicit def adtDecoder[A: Decoder, B: Decoder]: Decoder[ADT[A, B]] = ...

Now if I create a Bar, I don't get to have a codec for it. I know I could introduce a second type parameter for Bar to mitigate this, but assume it comes from a third party library.

[hardmettle]

@travisbrown picking this up as part of LSUG meetup .

• (Harsh)