Add jsonReader and jsonWriter functions for constructing readers and writers for case classes

[psliwa]

Hi,

There are functions like jsonFormat3 that create JsonFormat objects for case classes. Sometimes you need only readers or writers, because you are developing client or server side of an api. There is no problem, JsonFormat extends both. But there are situations when you have your type with custom serialisation logic, eg. serialising sealed trait, and you are developing eg. only api server side. So in such situations it would be good to define only writers for your's case classes in order to avoid defining unnecessary read function for your exemplary sealed trait. This PR provides such functions, analogous to jsonFormat familly.

This PR doesn't provide any new tests, because jsonFormat functions have been refactored and they use new functions internally, so ProductFormatsSpec tests new functions as well.

[janjaali]

Is there any possibility that this PR will be processed any further?

[jrudolph]

Is there any possibility that this PR will be processed any further?

I think it is looking quite nice. I'm trying to prepare a release in the upcoming months but it might still be some time as I have very little time to make progress.