Introduce a flag to default toEncoding to Generic rather than an implementation in terms of toJSON

[TeofilC]

This adds a flag (alternative names appreciated) that defines the default toEncoding method of ToJSON in terms of Generics rather than the toJSON method. This introduces symmetry between the toJSON and toEncoding methods, and makes it a lot easier to try to eliminate uses of toJSON from a codebase.

This change is placed behind a flag as it would break code that doesn't use Generic deriving and instead only gives an implementation of toJSON.

[phadej]

I'd rather introduce a newtype to use DerivingVia as mentioned in #933

Flag will change behavior for everyone, and that's quite a sledgehammer approach.

[TeofilC]

This flag would be off by default. But if your preference is to go for DerivingVia that makes sense.

I would still like a flag that disables the default instance of toEncoding in terms of toJSON though as it makes it viable to find cases where a toEncoding instance was missed. This would be off be default. What do you think of that?

[phadej]

My (#933) long term plan is to remove default implementation all-together.

Current default implementation is broken, but changing it doesn't make sense (unnecessary "breakage" in between) as DerivingVia will be more explicit and configurable.

[TeofilC]

Sure that makes sense and I agree is the right approach.
If you're not interested in having a flag for this in the interm I'll close this PR.