Add support for patching case classes with JSON

[travisbrown]

As requested by @vkostyukov, this addition supports the derivation of case class "updaters":

scala> import io.circe.generic.auto._, io.circe.jawn._
import io.circe.generic.auto._
import io.circe.jawn._

scala> case class Foo(i: Int, xs: List[String])
defined class Foo

scala> val foo = Foo(0, Nil)
foo: Foo = Foo(0,List())

scala> decode[Foo => Foo]("""{ "i": 13 }""").map(_(foo))
res0: cats.data.Xor[io.circe.Error,Foo] = Right(Foo(13,List()))

scala> decode[Foo => Foo]("""{ "xs": ["a", "b"] }""").map(_(foo))
res1: cats.data.Xor[io.circe.Error,Foo] = Right(Foo(0,List(a, b)))

scala> decode[Foo => Foo]("""{ "xs": ["a", "b"], "i": 1 }""").map(_(foo))
res2: cats.data.Xor[io.circe.Error,Foo] = Right(Foo(1,List(a, b)))

This is closely related to "incomplete" decoder derivation and the motivation is similar: we want to avoid lots of case classes like PatchedTodo here:

case class Todo(id: String, title: String, completed: Boolean, order: Int)
case class PatchedTodo(title: Option[String], completed: Option[Boolean], order: Option[Int])

Decoder[Foo => Foo] lets us accomplish the same thing without an extra boilerplate-y case class.

[vkostyukov]

Omg! This is awesome! Thanks a lot.

Mind publishing a snapshot version? I'd love to use this feature in the workshop.

[codecov-io]

Current coverage is 57.14%

Merging #24 into master will increase coverage by +0.01% as of 2accfd5

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[travisbrown]

@vkostyukov Yep, can definitely publish a snapshot. I also just found a few minor bugs in some of the cursor operations (a couple carried over from Argonaut and a couple of my own invention). These don't interfere with "normal" usage, but I might still try to pull together an 0.2.0 tonight. I'm stuck in LAX for two more hours thanks to another delayed plane, so I've got plenty of time…