Circe Encoders and Decoders Duplication

```implicit def decodeInstant: Decoder[Instant] = j8time.decodeInstant.or(decodeLong.map(Instant.ofEpochMilli))```

We don't actually need to generate these, these come free if we extend `TimeInstances`

https://github.com/circe/circe/blob/master/modules/java8/src/main/scala/io/circe/java8/time/TimeInstances.scala#L28

Also I can see this being a potential problem for some people since this requires an extra dep from circe.
Just a heads up!