Add codecs for singleton types about circe

[travisbrown]

Are there practical use cases for this? The refined module makes sense to me, but this seems to duplicate the role of other (less clunky) validation mechanisms.

I agree that this doesn't seem to fit nicely in generic, and while it'd only take a very simple macro to add it to core, I'm not sure we want to give up circe's macro-free status for something without a clear payoff.

[alexarchambault]

I typically use these when the underlying protocol expects a constant value. This allows to put / expect constant values in JSON objects, while still using auto-codecs.

[fthomas]

If you're going to add support for refined, you could accomplish the same with

case class CC(
  i: Int,
  message: String @@ Equal[Witness.`"test"`.T]
)

[alexarchambault]

@fthomas I agree, it's nice to have the simple Witness."test".T too.

I don't see it as a validation mechanism... It's just a way to put constants in the output.

These would just be codecs for singleton types, that are around anyway.

[travisbrown]

This is resolved by #162, which is now in master.