Unwanted object encoding instances for tuples, lists, etc. about circe

[lukewyman]

@travisbrown I'd like to take this one if it's still available. Might be a bit of a stretch at my current skill level, but I think I understand the problem in a vague/abstract way.

[travisbrown]

@lukewyman The situation is likely to change soon in #164, largely because the new case object encoding style option means that the generically derived encoders no longer always return JSON objects:

scala> import io.circe.Encoder, io.circe.generic.auto._, io.circe.syntax._
import io.circe.Encoder
import io.circe.generic.auto._
import io.circe.syntax._

scala> sealed trait Base; case object Foo extends Base; case object Bar extends Base
defined trait Base
defined object Foo
defined object Bar

scala> (Foo: Base).asJsonConfigured[io.circe.generic.config.CaseObjectString]
res0: io.circe.Json = "Foo"

This has been one of the most requested configuration options, and the extra guarantees of the ObjectEncoder type class didn't seem useful enough not to provide it.

We'll keep ObjectEncoder around, but the fact that the default generic derivation mechanism isn't using it means that we won't have to worry about this issue as much.

[hamnis]

Closing in cleanup run, If anyone cares about this, please comment and I'll reopen.