Add Codec type class (#133)

modules/core/shared/src/main/scala/io/circe/Codec.scala

@@ -0,0 +1,17 @@
+import io.circe._
+
+/**
+ * A type class that provides back and forth conversion between values of type `A`
+ * and the [[Json]] format. Must obey the laws defined in [[io.circe.tests.CodecLaws]].
+ */
+trait Codec[A] extends Encoder[A] with Decoder[A]
+
+object Codec {
+  def apply[A](implicit instance: Codec[A]): Codec[A] = instance
+
+  implicit def fromEncoderDecoder[A](implicit e: Encoder[A], d: Decoder[A]): Codec[A] =
+    new Codec[A] {
+      def apply(c: HCursor): Decoder.Result[A] = d(c)
+      def apply(a: A): Json = e(a)
+    }
+}

modules/core/shared/src/main/scala/io/circe/Decoder.scala

@@ -23,6 +23,9 @@ import scala.collection.immutable.{ Map => ImmutableMap, Set, SortedMap, SortedS
 import scala.collection.mutable.Builder
 import scala.util.{ Failure, Success, Try }
 
+/**
+ * A type class that provides a way to produce a value of type `A` from a [[Json]] value.
+ */
 trait Decoder[A] extends Serializable { self =>
 
   /**